Semiconductor device and manufacturing method thereof

ABSTRACT

To provide a transistor having high field effect mobility. To provide a transistor having stable electrical characteristics. To provide a transistor having low off-state current (current in an off state). To provide a semiconductor device including the transistor. The semiconductor device includes a semiconductor; a source electrode and a drain electrode including regions in contact with a top surface and side surfaces of the semiconductor; a gate insulating film including a region in contact with the semiconductor; and a gate electrode including a region facing the semiconductor with the gate insulating film provided therebetween. A length of a region of the semiconductor, which is not in contact with the source and drain electrodes, is shorter than a length of a region of the semiconductor, which is in contact with the source and drain electrodes, in a channel width direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, and a processor. Another embodiment of the present invention relates to a method for manufacturing a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, or a processor. Another embodiment of the present invention relates to a method for driving a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, or a processor. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, or a light-emitting device each including an oxide semiconductor.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, a light-emitting device, a lighting device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.

2. Description of the Related Art

As silicon which is used as a semiconductor of a transistor, either amorphous silicon or polycrystalline silicon is used depending on the purpose. For example, in the case of a transistor included in a large-sized display device, it is preferred to use amorphous silicon, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferred to use polycrystalline silicon, which can form a transistor having high field-effect mobility. As a method for forming polycrystalline silicon, high-temperature heat treatment or laser light treatment which is performed on amorphous silicon has been known.

In recent years, an oxide semiconductor has attracted attention. For example, a transistor which includes an amorphous oxide semiconductor containing indium, gallium, and zinc is disclosed (see Patent Document 1).

An oxide semiconductor can be formed by a sputtering method or the like, and thus can be used for a channel formation region of a transistor in a large display device. A transistor including an oxide semiconductor has high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including amorphous silicon can be retrofitted and utilized.

A transistor including an oxide semiconductor is known to have extremely small leakage current in an off state. For example, a CPU or the like with low-power consumption utilizing the leakage current of the transistor including an oxide semiconductor is disclosed (see Patent Document 2).

Patent Document 3 discloses that a transistor having high field-effect mobility can be obtained by a well potential formed using an active layer formed of a semiconductor.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2006-165528 -   [Patent Document 2] Japanese Published Patent Application No.     2012-257187 -   [Patent Document 3] Japanese Published Patent Application No.     2012-59860

SUMMARY OF THE INVENTION

An object is to provide a transistor having high field-effect mobility. Another object is to provide a transistor having stable electrical characteristics. Another object is to provide a transistor having low off-state current (current in an off state). Another object is to provide a semiconductor device including the transistor. Another object is to provide a novel semiconductor device.

Note that the description of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

According to one embodiment of the present invention, a semiconductor device includes a semiconductor; a source electrode and a drain electrode including a region in contact with a top surface of the semiconductor and a region in contact with side surfaces of the semiconductor; a gate insulating film including a region in contact with the semiconductor; and a gate electrode including a region facing the semiconductor with the gate insulating film provided therebetween. A length of a region of the semiconductor, which is not in contact with the source and drain electrodes, is shorter than a length of a region of the semiconductor, which is in contact with the source and drain electrodes, in a channel width direction.

According to one embodiment of the present invention, a thickness of a region of the semiconductor in the above semiconductor device, which is not in contact with the source electrode and the drain electrode, may be smaller than a region of the semiconductor, which is in contact with the source electrode and the drain electrode.

According to one embodiment of the present invention, the semiconductor in the above semiconductor device may be an oxide semiconductor.

According to one embodiment of the present invention, a method for manufacturing a semiconductor device including a semiconductor, a source electrode and a drain electrode including regions in contact with a top surface and side surfaces of the semiconductor, a gate insulating film including a region in contact with the semiconductor, and a gate electrode including a region facing the semiconductor with the gate insulating film provided therebetween, including the steps of forming a conductive film covering the semiconductor and dividing the conductive film over the semiconductor to form the source electrode and the drain electrode including the regions in contact with the top surface and the side surfaces of the semiconductor. By using the source electrode and the drain electrode as a mask, the thickness of a region of the semiconductor, which is not in contact with the source electrode and the drain electrode, is made small and the length of the region in the channel width direction is short.

It is possible to provide a transistor having high field-effect mobility. It is possible to provide a transistor having stable electrical characteristics. It is possible to provide a transistor with low off-state current. It is possible to provide a semiconductor device including the transistor. It is possible to provide a novel semiconductor device or the like. Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the objects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a top view and cross-sectional views which illustrate a transistor of one embodiment of the present invention;

FIGS. 2A and 2B are a top view and a cross-sectional view which illustrate a transistor of one embodiment of the present invention;

FIGS. 3A and 3B are a top view and a cross-sectional view which illustrate the transistor of one embodiment of the present invention;

FIGS. 4A and 4B are a top view and a cross-sectional view which illustrate the transistor of one embodiment of the present invention;

FIGS. 5A to 5C are a top view and cross-sectional views which illustrate the transistor of one embodiment of the present invention;

FIGS. 6A and 6B are a top view and a cross-sectional view which illustrate the transistor of one embodiment of the present invention;

FIGS. 7A to 7D are cross-sectional views and circuit diagrams of semiconductor devices of embodiments of the present invention;

FIGS. 8A and 8B are each a circuit diagram of a memory device of one embodiment of the present invention;

FIG. 9 is a block diagram of an RFID tag of one embodiment of the present invention;

FIGS. 10A to 10F show application examples of an RFID tag of one embodiment of the present invention;

FIG. 11 is a block diagram illustrating a CPU of one embodiment of the present invention;

FIG. 12 is a circuit diagram of a memory element of one embodiment of the present invention;

FIGS. 13A to 13C are a top view and circuit diagrams of a display device of one embodiment of the present invention;

FIG. 14 illustrates a display module of one embodiment of the present invention;

FIGS. 15A to 15F each illustrate an electronic device according to one embodiment of the present invention;

FIGS. 16A and 16B illustrate a structure used for calculation;

FIGS. 17A and 17B show Vg-Id characteristics obtained by calculation;

FIG. 18 shows current density distributions obtained by calculation;

FIGS. 19A and 19B show nanobeam electron diffraction patterns of oxide semiconductors;

FIGS. 20A and 20B illustrate an example of a transmission electron diffraction measurement apparatus; and

FIG. 21 shows an example of a structure analysis by transmission electron diffraction measurement.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways. Therefore, the present invention is not construed as being limited to description of the embodiments. In describing structures of the present invention with reference to the drawings, common reference numerals are used for the same portions in different drawings. Note that the same hatched pattern is applied to similar parts, and the similar parts are not especially denoted by reference numerals in some cases.

Note that the size, the thickness of films (layers), or regions in diagrams is sometimes exaggerated for simplicity.

A voltage usually refers to a potential difference between a given potential and a reference potential (e.g., a source potential or a ground potential (GND)). A voltage can be referred to as a potential and vice versa.

Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps or the stacking order of layers. Therefore, for example, description can be made even when “first” is replaced with “second”, “third”, or the like as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as the ordinal numbers used to specify one embodiment of the present invention.

Note that a “semiconductor” includes characteristics of an “insulator” in some cases when the conductivity is sufficiently low, for example. Furthermore, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “insulator” is not clear. Accordingly, a “semiconductor” in this specification can be called an “insulator” in some cases. Similarly, an “insulator” in this specification can be called a “semiconductor” in some cases.

Furthermore, a “semiconductor” includes characteristics of a “conductor” in some cases when the conductivity is sufficiently high, for example. Furthermore, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “conductor” is not clear. Accordingly, a “semiconductor” in this specification can be called a “conductor” in some cases. Similarly, a “conductor” in this specification can be called a “semiconductor” in some cases.

Note that an impurity in a semiconductor refers to, for example, elements other than the main components of a semiconductor. For example, an element with a concentration of lower than 0.1 atomic % is an impurity. When an impurity is contained, the density of states (DOS) may be formed in a semiconductor, the carrier mobility may be decreased, or the crystallinity may be decreased, for example. When the semiconductor is an oxide semiconductor, examples of an impurity which changes the characteristics of the semiconductor include Group 1 elements, Group 2 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components; specifically, there are hydrogen (including water), lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen, for example. When the semiconductor is an oxide semiconductor, oxygen vacancies may be formed by entry of impurities such as hydrogen, for example. Furthermore, when the semiconductor is silicon, examples of an impurity which changes the characteristics of the semiconductor include oxygen, Group 1 elements except hydrogen, Group 2 elements, Group 13 elements, and Group 15 elements.

<Transistor Structure>

The structures of transistors of embodiments of the present invention will be described below.

<Transistor Structure 1>

FIGS. 1A and 1B are a top view and a cross-sectional view which illustrate a transistor of one embodiment of the present invention. FIG. 1A is a top view and FIG. 1B is a cross-sectional view taken along dashed-dotted line A1-A2 and dashed-dotted line A3-A4 in FIG. 1A. Note that for simplification of the drawing, some components are not illustrated in the top view in FIG. 1A.

The transistor in FIGS. 1A and 1B includes an insulating film 402 having a projection over a substrate 400, a semiconductor 406 over the projection of the insulating film 402, a conductive film 416 a and a conductive film 416 b in contact with a top surface and side surfaces of the semiconductor 406, an insulating film 412 over the semiconductor 406, the conductive film 416 a, and the conductive film 416 b, a conductive film 404 which is in contact with a top surface of the insulating film 412 and faces the top surface and the side surfaces of the semiconductor 406, and an insulating film 418 over the conductive film 416 a, the conductive film 416 b, and the conductive film 404. Note that the insulating film 402 does not necessarily include a projection. The conductive film 404 serves as a gate electrode of the transistor. The insulating film 412 functions as a gate insulating film of the transistor. The conductive film 416 a and the conductive film 416 b serve as a source electrode and a drain electrode of the transistor.

As illustrated in FIG. 1B, the thickness of a region of the semiconductor 406, which overlaps with the conductive film 416 a and the conductive film 416 b, is larger than that of a region of the semiconductor 406, which does not overlap with the conductive film 416 a and the conductive film 416 b. In other words, the thickness of a region of the semiconductor 406, which does not overlap with the conductive film 416 a and the conductive film 416 b, is smaller than that of a region of the semiconductor 406, which overlaps with the conductive film 416 a and the conductive film 416 b.

The length of a region of the semiconductor 406, which overlaps with the conductive film 416 a and the conductive film 416 b, is longer than that of a region of the semiconductor 406, which does not overlap with the conductive film 416 a and the conductive film 416 b, in the dashed-dotted line A3-A4 direction. In other words, the length of a region of the semiconductor 406, which does not overlap with the conductive film 416 a and the conductive film 416 b, is shorter than that of a region of the semiconductor 406, which overlaps with the conductive film 416 a and the conductive film 416 b, in the dashed-dotted line A3-A4 direction.

When attention is focused on a channel formation region of the transistor, the dashed-dotted line A3-A4 direction can be referred to as the channel width direction. The channel width direction may be referred to as the short-side direction, the width direction, or the lateral direction. Similarly, the dashed-dotted line A1-A2 direction can be referred to as the channel length direction. The channel length direction may be referred to as the long-side direction or the longitudinal direction.

Note that the channel length refers to a distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor and a gate electrode overlap with each other in a top view. In other words, in FIG. 1A, the channel length refers to a distance between the conductive film 416 a and the conductive film 416 b in a region where the semiconductor 406 and the conductive film 404 overlap with each other. A channel width refers to a length of a portion where a source faces and is parallel to a drain and where a semiconductor and a gate electrode overlap with each other. In other words, in FIG. 1A, the channel width refers to a length of a portion where the conductive film 416 a faces and is parallel to the conductive film 416 b and where the semiconductor 406 and the conductive film 404 overlap with each other.

As described above, the semiconductor illustrated in FIGS. 1A to 1C has a large region in contact with the conductive film 416 a and the conductive film 416 b and a small region (channel formation region) not in contact with the conductive film 416 a and the conductive film 416 b (a dumbbell shape or a gourd shape). Thus, an electric field is easily applied from the gate electrode to the entire channel formation region and contact resistance to the source electrode and the drain electrode can be decreased. In other words, the transistor illustrated in FIGS. 1A to 1C has high on-state current because the resistance in an on state (also referred to as on-state resistance) is low and has low off-state current because the resistance in an off state (also referred to as off-state resistance) is high.

The semiconductor 406 can be electrically surrounded by an electric field of the conductive film 404 (a structure in which a semiconductor is electrically surrounded by an electric field of a conductive film is referred to as a surrounded channel (s-channel) structure). Therefore, a channel is formed in the entire semiconductor 406 (bulk) in some cases. In the s-channel structure, a large amount of current can flow between a source and a drain of a transistor, so that a high on-state current can be obtained.

The s-channel structure is suitable for a miniaturized transistor because a high on-state current can be obtained. A semiconductor device including the miniaturized transistor can have a high integration degree and high density. For example, the channel length of the transistor is preferably less than or equal to 40 nm, more preferably less than or equal to 30 nm, still more preferably less than or equal to 20 nm and the channel width of the transistor is preferably less than or equal to 40 nm, more preferably less than or equal to 30 nm, still more preferably less than or equal to 20 nm.

At least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is provided on at least part (or all) of a surface, a side surface, a top surface, and/or a bottom surface of a semiconductor, e.g., the semiconductor 406.

Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is in contact with at least part (or all) of a surface, a side surface, a top surface, and/or a bottom surface of a semiconductor, e.g., the semiconductor 406. Further alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is in contact with at least part (or all) of a semiconductor, e.g., the semiconductor 406.

Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is electrically connected to at least part (or all) of a surface, a side surface, a top surface, and/or a bottom surface of a semiconductor, e.g., the semiconductor 406. Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is electrically connected to at least part (or all) of a semiconductor, e.g., the semiconductor 406.

Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is provided near at least part (or all) of a surface, a side surface, a top surface, and/or a bottom surface of a semiconductor, e.g., the semiconductor 406. Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is provided near at least part (or all) of a semiconductor, e.g., the semiconductor 406.

Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is provided on a side of at least part (or all) of a surface, a side surface, a top surface, and/or a bottom surface of a semiconductor, e.g., the semiconductor 406. Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is provided on a side of at least part (or all) of a semiconductor, e.g., the semiconductor 406.

Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is provided obliquely above at least part (or all) of a surface, a side surface, a top surface, and/or a bottom surface of a semiconductor, e.g., the semiconductor 406. Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is provided obliquely above at least part (or all) of a semiconductor, e.g., the semiconductor 406.

Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is provided above at least part (or all) of a surface, a side surface, a top surface, and/or a bottom surface of a semiconductor, e.g., the semiconductor 406. Alternatively, at least part (or all) of the conductive film 416 a (and/or the conductive film 416 b) is provided above at least part (or all) of a semiconductor, e.g., the semiconductor 406.

There is no large limitation on the substrate 400. For example, a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or a stabilized zirconia substrate (e.g., an yttria-stabilized zirconia substrate) may be used. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, a silicon-on-insulator (SOI) substrate, or the like may be used. Still alternatively, any of these substrates provided with a semiconductor element may be used.

The insulating film 402 may be formed of, for example, a single layer or a stack of an insulating film containing aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide.

As the semiconductor 406, a Group 14 semiconductor film such as a silicon film or a germanium film, a compound semiconductor film such as a silicon carbide film, a germanium silicide film, a gallium arsenide film, an indium phosphide film, a zinc selenide film, a cadmium sulfide film, or an oxide semiconductor film, an organic semiconductor film, or the like may be used. The semiconductor 406 may have a single-layer structure or a stacked-layer structure.

An oxide semiconductor is preferably used as the semiconductor 406. Specific examples of the oxide semiconductor are described later.

The conductive film 416 a and the conductive film 416 b each may be formed to have a single-layer structure or a stacked-layer structure using a conductive film containing one or more kinds of aluminum, titanium, chromium, cobalt, nickel, copper, yttrium, zirconium, molybdenum, ruthenium, silver, tantalum, and tungsten, for example.

The insulating film 412 may be formed of, for example, a single layer or a stack of an insulating film containing aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide.

The conductive film 404 may be formed to have a single-layer structure or a stacked-layer structure using a conductive film containing one or more kinds of aluminum, titanium, chromium, cobalt, nickel, copper, yttrium, zirconium, molybdenum, ruthenium, silver, tantalum, and tungsten, for example.

The insulating film 418 may be formed of, for example, a single layer or a stack of an insulating film containing aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide.

The insulating film 402 can have a function of preventing diffusion of impurities from the substrate 400. Here, in the case where the semiconductor 406 is an oxide semiconductor, the insulating film 402 can have a function of supplying oxygen to the semiconductor 406. Therefore, the insulating film 402 is preferably an insulating film containing oxygen. For example, an insulating film containing oxygen more than that in the stoichiometric composition is preferable.

<Oxide Semiconductor>

An oxide semiconductor which can be used as the semiconductor 406 is described in detail below.

The oxide semiconductor which can be used as the semiconductor 406 is an oxide containing indium. An oxide can have high carrier mobility (electron mobility) by containing indium, for example. An oxide semiconductor preferably contains an element M. The element M is preferably aluminum, gallium, yttrium, tin, or the like. Other elements which can be used as the element M are boron, silicon, titanium, iron, nickel, germanium, yttrium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and the like. Note that two or more of the above elements may be used in combination as the element M. The element M is an element having high bonding energy with oxygen, for example. The element M is an element that can increase the energy gap of the oxide, for example. Furthermore, the oxide semiconductor preferably contains zinc. When the oxide contains zinc, the oxide is easily to be crystallized, for example.

Note that the oxide semiconductor is not limited to the oxide containing indium. The oxide semiconductor may be, for example, zinc tin oxide or gallium tin oxide.

For the oxide semiconductor, an oxide with a wide energy gap is used. For example, the energy gap of the oxide semiconductor is greater than or equal to 2.5 eV and less than or equal to 4.2 eV, preferably greater than or equal to 2.8 eV and less than or equal to 3.8 eV, more preferably greater than or equal to 3 eV and less than or equal to 3.5 eV.

An influence of impurities in the oxide semiconductor is described below. In order to obtain stable electrical characteristics of a transistor, it is effective to reduce the concentration of impurities in the oxide semiconductor to have lower carrier density so that the oxide semiconductor is highly purified. The carrier density of the oxide semiconductor is set to be lower than 1×10¹⁷/cm³, lower than 1×10¹⁵/cm³, or lower than 1×10¹³/cm³. In order to reduce the concentration of impurities in the oxide semiconductor, the concentration of impurities in a film which is adjacent to the oxide semiconductor is preferably reduced.

For example, silicon in the oxide semiconductor might serve as a carrier trap or a carrier generation source. Therefore, the concentration of silicon in a region between the oxide semiconductor and the insulating film 402 measured by secondary ion mass spectrometry (SIMS) is set to be lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, further preferably lower than 2×10¹⁸ atoms/cm³. The concentration of silicon in a region between the oxide semiconductor and the insulating film 412 measured by SIMS is set to be lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, further preferably lower than 2×10¹⁸ atoms/cm³.

Furthermore, when hydrogen is contained in the oxide semiconductor, the carrier density is increased in some cases. The concentration of hydrogen in the oxide semiconductor measured by SIMS is set to be lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³, still further preferably lower than or equal to 5×10¹⁸ atoms/cm³. When nitrogen is contained in the oxide semiconductor, the carrier density is increased in some cases. The concentration of nitrogen in the oxide semiconductor measured by SIMS is set to be lower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³, still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

It is preferable to reduce the concentration of hydrogen in the insulating film 402 in order to reduce the concentration of hydrogen in the oxide semiconductor. The concentration of hydrogen in the insulating film 402 measured by SIMS is set to be lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³, still further preferably lower than or equal to 5×10¹⁸ atoms/cm³. It is preferable to reduce the concentration of nitrogen in the insulating film 402 in order to reduce the concentration of nitrogen in the oxide semiconductor. The concentration of nitrogen in the insulating film 402 measured by SIMS is set to be lower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³, still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

It is preferable to reduce the concentration of hydrogen in the insulating film 412 in order to reduce the concentration of hydrogen in the oxide semiconductor. The concentration of hydrogen in the insulating film 412 measured by SIMS is set to be lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³, still further preferably lower than or equal to 5×10¹⁸ atoms/cm³. It is preferable to reduce the concentration of nitrogen in the insulating film 412 in order to reduce the concentration of nitrogen in the oxide semiconductor. The concentration of nitrogen in the insulating film 412 measured by SIMS is set to be lower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³, still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

The structure of an oxide semiconductor is described below.

Oxide semiconductors are classified roughly into a single-crystal oxide semiconductor and a non-single-crystal oxide semiconductor. The non-single-crystal oxide semiconductor includes any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a microcrystalline oxide semiconductor, an amorphous oxide semiconductor, and the like.

First, a CAAC-OS is described.

The CAAC-OS is one of oxide semiconductors including a plurality of crystal parts, and most of the crystal parts each fit inside a cube whose one side is less than 100 nm. Thus, there is a case where a crystal part included in the CAAC-OS fits inside a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm.

In a transmission electron microscope (TEM) image of the CAAC-OS, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflecting a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS is formed is referred to as a formation surface) or a top surface of the CAAC-OS, and is arranged in parallel to the formation surface or the top surface of the CAAC-OS.

On the other hand, according to the TEM image of the CAAC-OS observed in a direction substantially perpendicular to the sample surface (plan-view TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts.

Note that in an electron diffraction pattern of the CAAC-OS, spots (luminescent spots) having alignment are shown. For example, when electron diffraction with an electron beam having a diameter of 1 nm or more and 30 nm or less (such electron diffraction is also referred to as nanobeam electron diffraction) is performed on the top surface of the CAAC-OS film, spots are observed (see FIG. 19A).

From the results of the cross-sectional TEM image and the plan-view TEM image, alignment is found in the crystal parts in the CAAC-OS.

A CAAC-OS is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS including an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS.

In this specification, the trigonal and rhombohedral crystal systems are included in the hexagonal crystal system.

On the other hand, when the CAAC-OS is analyzed by an in-plane method in which an X-ray enters a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal. Here, analysis (φ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (φ axis) with 2θ fixed at around 56°. In the case where the sample is a single-crystal oxide semiconductor of InGaZnO₄, six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. On the other hand, in the case of a CAAC-OS, a peak is not clearly observed even when φ scan is performed with 2θ fixed at around 56°.

According to the above results, in the CAAC-OS having c-axis alignment, while the directions of a-axes and b-axes are different between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of the CAAC-OS or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, for example, in the case where a shape of the CAAC-OS is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS.

Furthermore, the degree of crystallinity in the CAAC-OS is not necessarily uniform. For example, in the case where crystal growth leading to the CAAC-OS occurs from the vicinity of the top surface of the film, the degree of the crystallinity in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Furthermore, when an impurity is added to the CAAC-OS, the crystallinity in a region to which the impurity is added is changed, and the degree of crystallinity in the CAAC-OS varies depending on regions.

Note that when the CAAC-OS with an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak of 2θ may also be observed at around 36°, in addition to the peak of 2θ at around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS. It is preferable that in the CAAC-OS, a peak of 2θ appear at around 31° and a peak of 2θ not appear at around 36°.

The CAAC-OS is an oxide semiconductor with a low impurity concentration. The impurity means here an element other than main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element such as silicon has a higher strength to bond with oxygen than that of a metal element included in the oxide semiconductor. Therefore, when the element takes oxygen away in the oxide semiconductor, the atomic arrangement in the oxide semiconductor is disrupted, whereby the crystallinity of the oxide semiconductor is lowered in some cases. In addition, a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disrupts the atomic arrangement in the oxide semiconductor, whereby the crystallinity of the oxide semiconductor is lowered in some cases. Note that the impurity included in the oxide semiconductor might serve as a carrier trap or a carrier generation source.

The CAAC-OS is an oxide semiconductor having a low density of defect states. In some cases, oxygen vacancies in the oxide semiconductor serve as carrier traps or carrier generation sources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) is referred to as a “highly purified intrinsic” or “substantially highly purified intrinsic” state. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier generation sources, and thus has a low carrier density in some cases. Thus, a transistor including the oxide semiconductor rarely has a negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier traps. Accordingly, the transistor including the oxide semiconductor has little variation in electrical characteristics and high reliability. Charges trapped by the carrier traps in the oxide semiconductor take a long time to be released. The trapped charges may behave like fixed electric charges. Thus, the transistor which includes the oxide semiconductor having high impurity concentration and a high density of defect states has unstable electrical characteristics in some cases.

With the use of the CAAC-OS in a transistor, variation in the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor is described.

In an image obtained with the TEM, crystal parts cannot be found clearly in the microcrystalline oxide semiconductor in some cases. In most cases, the size of a crystal part in the microcrystalline oxide semiconductor is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. A microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm, or a size greater than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as nanocrystal (nc). An oxide semiconductor film including nanocrystal is referred to as an nc-OS (nanocrystalline oxide semiconductor). In an image obtained with TEM, a grain boundary cannot be found clearly in the nc-OS in some cases.

In the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic order. Furthermore, there is no regularity of crystal orientation between different crystal parts in the nc-OS; thus, the orientation of the whole nc-OS is not observed. Accordingly, in some cases, the nc-OS cannot be distinguished from an amorphous oxide semiconductor depending on an analysis method. For example, when the nc-OS is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than the diameter of a crystal part, a peak which shows a crystal plane does not appear. Furthermore, a halo pattern is shown in an electron diffraction pattern (also referred to as a selected area electron diffraction pattern) of the nc-OS obtained by using an electron beam having a probe diameter (e.g., larger than or equal to 50 nm) larger than the diameter of a crystal part. Meanwhile, spots are shown in a nanobeam electron diffraction pattern of the nc-OS obtained by using an electron beam having a probe diameter close to, or smaller than the diameter of a crystal part. Furthermore, in a nanobeam electron diffraction pattern of the nc-OS, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS, a plurality of spots are shown in a ring-like region in some cases (see FIG. 19B).

The nc-OS is an oxide semiconductor having more regularity than an amorphous oxide semiconductor. Therefore, the nc-OS has a lower density of defect states than an amorphous oxide semiconductor. However, there is no regularity of crystal orientation between different crystal parts in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS.

Note that an oxide semiconductor may include two or more of an amorphous oxide semiconductor, a microcrystalline oxide semiconductor, and a CAAC-OS, for example.

In the case where the oxide semiconductor has a plurality of structures, the structures can be analyzed using nanobeam electron diffraction in some cases.

FIG. 20A illustrates a transmission electron diffraction measurement apparatus which includes an electron gun chamber 10, an optical system 12 below the electron gun chamber 10, a sample chamber 14 below the optical system 12, an optical system 16 below the sample chamber 14, an observation chamber 20 below the optical system 16, a camera 18 installed in the observation chamber 20, and a film chamber 22 below the observation chamber 20. The camera 18 is provided to face toward the inside of the observation chamber 20. Note that the film chamber 22 is not necessarily provided.

FIG. 20B illustrates an internal structure of the transmission electron diffraction measurement apparatus illustrated in FIG. 20A. In the transmission electron diffraction measurement apparatus, a substance 28 which is positioned in the sample chamber 14 is irradiated with electrons emitted from an electron gun installed in the electron gun chamber 10 through the optical system 12. Electrons passing through the substance 28 enter a fluorescent plate 32 provided in the observation chamber 20 through the optical system 16. On the fluorescent plate 32, a pattern corresponding to the intensity of the incident electron appears, which allows measurement of a transmission electron diffraction pattern.

The camera 18 is installed so as to face the fluorescent plate 32 and can take a picture of a pattern appearing in the fluorescent plate 32. An angle formed by a straight line which passes through the center of a lens of the camera 18 and the center of the fluorescent plate 32 and an upper surface of the fluorescent plate 32 is, for example, 15° or more and 80° or less, 30° or more and 75° or less, or 45° or more and 70° or less. As the angle is reduced, distortion of the transmission electron diffraction pattern taken by the camera 18 becomes larger. Note that if the angle is obtained in advance, the distortion of an obtained transmission electron diffraction pattern can be corrected. Note that the film chamber 22 may be provided with the camera 18. For example, the camera 18 may be set in the film chamber 22 so as to be opposite to the incident direction of electrons 24. In this case, a transmission electron diffraction pattern with less distortion can be taken from the rear surface of the fluorescent plate 32.

A holder for fixing the substance 28 that is a sample is provided in the sample chamber 14. The holder transmits electrons passing through the substance 28. The holder may have, for example, a function of moving the substance 28 in the direction of the X, Y, and Z axes. The movement function of the holder may have an accuracy of moving the substance in the range of, for example, 1 nm to 10 nm, 5 nm to 50 nm, 10 nm to 100 nm, 50 nm to 500 nm, and 100 nm to 1 μm. The range is preferably determined to be an optimal range for the structure of the substance 28.

Then, a method for measuring a transmission electron diffraction pattern of a substance by the transmission electron diffraction measurement apparatus described above is described.

For example, changes in the structure of a substance can be observed by changing (scanning) the irradiation position of the electrons 24 that are a nanobeam in the substance, as illustrated in FIG. 20B. At this time, when the substance 28 is a CAAC-OS, a diffraction pattern as shown in FIG. 19A is observed. When the substance 28 is an nc-OS, a diffraction pattern shown in FIG. 19B is observed.

Even when the substance 28 is a CAAC-OS, a diffraction pattern similar to that of an nc-OS or the like is partly observed in some cases. Therefore, whether or not a CAAC-OS is favorable can be determined by the proportion of a region where a diffraction pattern of a CAAC-OS is observed in a predetermined area (also referred to as proportion of CAAC). In the case of a high quality CAAC-OS, for example, the proportion of CAAC is higher than or equal to 60%, preferably higher than or equal to 80%, further preferably higher than or equal to 90%, still further preferably higher than or equal to 95%. Note that a region where a diffraction pattern different from that of a CAAC-OS is observed is referred to as the proportion of not-CAAC.

For example, transmission electron diffraction patterns were obtained by scanning a top surface of a sample including a CAAC-OS obtained just after deposition (represented as “as-depo”) and a top surface of a sample including a CAAC-OS subjected to heat treatment at 350° C. or 450° C. Here, the proportion of CAAC was obtained in such a manner that diffraction patterns were observed by scanning for 60 seconds at a rate of 5 nm/second and the obtained diffraction patterns were converted into still images every 0.5 seconds. Note that as an electron beam, a nanometer-size electron beam with a probe diameter of 1 nm was used.

FIG. 21 shows the proportion of CAAC in each sample. The proportion of CAAC of the sample after heat treatment at 450° C. is high compared with those of the sample of as-depo and the sample after heat treatment at 350° C. That is, heat treatment at a high temperature (e.g., higher than or equal to 400° C.) reduces the proportion of non-CAAC (increases the proportion of CAAC). Here, most of diffraction patterns different from that of a CAAC-OS are diffraction patterns similar to that of an nc-OS. Therefore, the above results suggest that the region having a structure similar to that of an nc-OS is rearranged by the heat treatment owing to the influence of the structure of the adjacent region, whereby the region becomes CAAC.

With such a measurement method, the structure of an oxide semiconductor having a plurality of structures can be analyzed in some cases.

The oxide semiconductor may be a stack of oxide semiconductors. For example, the oxide semiconductor may have a two-layer structure or a three-layer structure.

For example, the case where the oxide semiconductor has a three-layer structure is described with reference to FIG. 1C.

For an oxide semiconductor layer 406 b (middle layer), the description of the above-described oxide semiconductor can be referred to. An oxide semiconductor layer 406 a (bottom layer) and an oxide semiconductor layer 406 c (top layer) include one or more elements other than oxygen included in the oxide semiconductor layer 406 b. Since the oxide semiconductor layer 406 a and the oxide semiconductor layer 406 c each include one or more elements other than oxygen included in the oxide semiconductor layer 406 b, an interface state is less likely to be formed at the interface between the oxide semiconductor layer 406 a and the oxide semiconductor layer 406 b and the interface between the oxide semiconductor layer 406 b and the oxide semiconductor layer 406 c.

In the case of using an In-M-Zn oxide as the oxide semiconductor layer 406 a, when summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be less than 50 atomic % and greater than or equal to 50 atomic %, respectively, further preferably less than 25 atomic % and greater than or equal to 75 atomic %, respectively. In the case of using an In-M-Zn oxide as the oxide semiconductor layer 406 b, when summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be greater than or equal to 25 atomic % and less than 75 atomic %, respectively, further preferably greater than or equal to 34 atomic % and less than 66 atomic %, respectively. In the case of using an In-M-Zn oxide as the oxide semiconductor layer 406 c, when summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be less than 50 atomic % and greater than or equal to 50 atomic %, respectively, further preferably less than 25 atomic % and greater than or equal to 75 atomic %, respectively. Note that the oxide semiconductor layer 406 c may be an oxide that is a type the same as that of the oxide semiconductor layer 406 a.

Here, in some cases, there is a mixed region of the oxide semiconductor layer 406 a and the oxide semiconductor layer 406 b between the oxide semiconductor layer 406 a and the oxide semiconductor layer 406 b. Furthermore, in some cases, there is a mixed region of the oxide semiconductor layer 406 b and the oxide semiconductor layer 406 c between the oxide semiconductor layer 406 b and the oxide semiconductor layer 406 c. The mixed region has a low interface state density. For that reason, the stack of the oxide semiconductor layer 406 a, the oxide semiconductor layer 406 b, and the oxide semiconductor layer 406 c has a band structure where energy at each interface and in the vicinity of the interface is changed continuously (continuous junction).

As the oxide semiconductor layer 406 b, an oxide having an electron affinity higher than those of the oxide semiconductor layers 406 a and 406 c is used. For example, as the oxide semiconductor layer 406 b, an oxide having an electron affinity higher than those of the oxide semiconductor layers 406 a and 406 c by 0.07 eV or higher and 1.3 eV or lower, preferably 0.1 eV or higher and 0.7 eV or lower, more preferably 0.15 eV or higher and 0.4 eV or lower is used. Note that the electron affinity refers to an energy difference between the vacuum level and the bottom of the conduction band.

At this time, when an electric field is applied to a gate electrode, a channel is formed in the oxide semiconductor layer 406 b having the highest electron affinity in the oxide semiconductor layer 406 a, the oxide semiconductor layer 406 b, and the oxide semiconductor layer 406 c.

Moreover, the thickness of the oxide semiconductor layer 406 c is preferably as small as possible to increase the on-state current of the transistor. The thickness of the oxide semiconductor layer 406 c is set to be less than 10 nm, preferably less than or equal to 5 nm, further preferably less than or equal to 3 nm, for example. Meanwhile, the oxide semiconductor layer 406 c has a function of blocking elements other than oxygen (such as silicon) included in the adjacent insulating film from entering the oxide semiconductor layer 406 b where a channel is formed. For this reason, it is preferable that the oxide semiconductor layer 406 c have a certain thickness. The thickness of the oxide semiconductor layer 406 c is set to be greater than or equal to 0.3 nm, preferably greater than or equal to 1 nm, further preferably greater than or equal to 2 nm, for example.

To improve reliability, preferably, the thickness of the oxide semiconductor layer 406 a is large and the thickness of the oxide semiconductor layer 406 c is small. Specifically, the thickness of the oxide semiconductor layer 406 a is set to be greater than or equal to 20 nm, preferably greater than or equal to 30 nm, further preferably greater than or equal to 40 nm, still further preferably greater than or equal to 60 nm. With the oxide semiconductor layer 406 a having a thickness greater than or equal to 20 nm, preferably greater than or equal to 30 nm, further preferably greater than or equal to 40 nm, still further preferably greater than or equal to 60 nm, the distance from the interface between the adjacent insulating film and the oxide semiconductor layer 406 a to the oxide semiconductor layer 406 b where the channel is formed can be greater than or equal to 20 nm, preferably greater than or equal to 30 nm, further preferably greater than or equal to 40 nm, still further preferably greater than or equal to 60 nm. Note that since the productivity of a semiconductor device might be reduced, the thickness of the oxide semiconductor layer 406 a is set to be less than or equal to 200 nm, preferably less than or equal to 120 nm, further preferably less than or equal to 80 nm.

For example, the concentration of silicon in a region between the oxide semiconductor layer 406 b and the oxide semiconductor layer 406 a measured by SIMS is set to be lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, further preferably lower than 2×10¹⁸ atoms/cm³. The concentration of silicon in a region between the oxide semiconductor layer 406 b and the oxide semiconductor layer 406 c measured by SIMS is set to be lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, further preferably lower than 2×10¹⁸ atoms/cm³.

It is preferable to reduce the concentration of hydrogen in the oxide semiconductor layer 406 a and the oxide semiconductor layer 406 c in order to reduce the concentration of hydrogen in the oxide semiconductor layer 406 b. The concentration of hydrogen in the oxide semiconductor layer 406 a and the oxide semiconductor layer 406 c measured by SIMS is set to be lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³, still further preferably lower than or equal to 5×10¹⁸ atoms/cm³. It is preferable to reduce the concentration of nitrogen in the oxide semiconductor layer 406 a and the oxide semiconductor layer 406 c in order to reduce the concentration of nitrogen in the oxide semiconductor layer 406 b. The concentration of nitrogen in the oxide semiconductor layer 406 a and the oxide semiconductor layer 406 c measured by SIMS is set to be lower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³, still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

The above three-layer structure is an example of the oxide semiconductor. For example, a two-layer structure without the oxide semiconductor layer 406 a or the oxide semiconductor layer 406 c may be employed.

<Manufacturing Method of Transistor Structure 1>

A method for manufacturing the transistor illustrated in FIGS. 1A to 1C will be described below with reference to FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 4A and 4B, and FIGS. 5A to 5C. Note that FIG. 2A, FIG. 3A, FIG. 4A, and FIG. 5A are top views of the transistor, and FIG. 2B, FIG. 3B, FIG. 4B, and FIG. 5B are cross-sectional views taken along dashed-dotted line A1-A2 and dashed-dotted line A3-A4.

First, the insulating film 402 is formed over the substrate 400. The insulating film 402 may be formed by a sputtering method, a chemical vapor deposition (CVD) method, a molecular beam epitaxy (MBE) method, a pulsed laser deposition (PLD) method, an atomic layer deposition (ALD) method, or the like.

Note that in the case where the insulating film 402 is a stacked-layer film, layers in the stacked-layer film may be formed using by different formation methods such as the above formation methods. For example, the first layer may be formed by a CVD method and the second layer may be formed by an ALD method. Alternatively, the first layer may be formed by a sputtering method and the second layer may be formed by an ALD method. When layers are formed by different formation methods as described above, the layers can have different functions or different properties. Furthermore, by stacking the layers, a more appropriate film can be formed as a stacked-layer film.

In other words, an n-th layer is formed by at least one of a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, and the like, and an n+1-th layer is formed by at least one of a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, and the like (n is a natural number). Note that the n-th layer and the n+1-th layer may be formed by the same formation method or different formation methods. Note that the n-th layer and an n+2-th layer may be formed by the same formation method. Alternatively, all the layers may be formed by the same formation method.

Alternatively, when a silicon substrate is used as the substrate 400, the insulating film to be the insulating film 402 can be formed by a thermal oxidation method.

Then, in order to planarize the surface of the insulating film to be the insulating film 402, chemical mechanical polishing (CMP) may be performed. By CMP treatment, the average surface roughness (Ra) of the insulating film to be the insulating film 402 is less than or equal to 1 nm, preferably less than or equal to 0.3 nm, more preferably less than or equal to 0.1 nm. In some cases, Ra that is less than or equal to the above value can increase the crystallinity of the semiconductor 406. Ra can be measured using an atomic force microscope (AFM).

Next, the semiconductor 406 is formed (see FIGS. 2A and 2B).

A semiconductor to be the semiconductor 406 may be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

In the case where the semiconductor to be the semiconductor 406 is etched to form the semiconductor 406, it is preferable that the etching be performed not to damage a surface of the semiconductor 406 to be processed. For example, neutral beam etching may be performed by a dry etching method. Since a neutral beam is used, charge-up is not caused owing to charges and the energy is low; thus, the etching can be performed with less damage. In the case where the semiconductor 406 is a crystal, a wet etching method utilizing different etching rates depending on crystal planes may be employed. By a wet etching method, damage to the surface to be processed can be reduced.

For example, by a CVD method, a semiconductor whose composition is gradually changed may be formed as the semiconductor 406.

Note that CVD methods can be classified into a plasma enhanced CVD (PECVD) method utilizing plasma, a thermal CVD (TCVD) method utilizing heat, and the like. The CVD methods can be further classified into a metal CVD (MCVD) method and a metal organic CVD (MOCVD) method according to a source gas to be used.

In the case of a PECVD method, a high quality film can be obtained at relatively low temperature. Since a TCVD method does not use plasma, plasma damage does not occur; thus a film with less defects can be obtained.

In the case of a CVD method, the composition of a film to be obtained can be controlled by adjusting the flow ratio of a source gas. For example, in the cases of an MCVD method and an MOCVD method, a film having given composition can be formed by adjusting the flow ratio of a source gas. For example, in the cases of an MCVD method and an MOCVD method, by changing the flow ratio of a source gas while deposition is performed, a film whose composition is gradually changed can be formed. In the case where a film is formed while the flow ratio of a source gas is changed, deposition time can be shortened compared with the case where a film is formed using a plurality of deposition chambers because time for transfer and time for pressure adjustment are not needed. Consequently, the productivity of a transistor can be increased. A specific example of a deposition apparatus by which an MOCVD method can be employed is described later.

Alternatively, a film whose composition is gradually changed may be formed by a sputtering method, an MBE method, a PLD method, or an ALD method.

The insulating film 402 is in contact with the semiconductor 406. When a semiconductor to be the semiconductor 406 is formed, a formation method by which the insulating film 402 is not damaged is preferably employed. In other words, the semiconductor is preferably formed by an MOCVD method, for example.

Note that in the case where the semiconductor 406 is formed to have a stacked-layer structure, layers in the stacked-layer film may be formed by different formation methods such as a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, and an ALD method. For example, the first layer may be formed by an MOCVD method and the second layer may be formed by a sputtering method. Alternatively, the first layer may be formed by an ALD method and the second layer may be formed by an MOCVD method. Alternatively, the first layer may be formed by an ALD method and the second layer may be formed by a sputtering method. Alternatively, the first layer may be formed by an ALD method, the second layer may be formed by a sputtering method, and the third layer may be formed by an ALD method. When layers are formed by different formation methods as described above, the layers can have different functions or different properties. Furthermore, by stacking the layers, a more appropriate film can be formed as a stacked-layer film.

In other words, in the case where the semiconductor 406 is formed to have a stacked-layer structure, for example, when an n-th layer is formed by at least one of a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, an ALD method, and the like and an n+1-th layer is formed by at least one of a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, an ALD method, and the like, the n-th layer and the n+1-th layer may be formed by different formation methods (n is a natural number). Note that the n-th layer and the n+2-th layer may be formed by the same formation method. Alternatively, all the layers may be formed by the same formation method.

Note that the semiconductor 406 or at least one layer in the stacked semiconductor 406 and the insulating film 402 or at least one layer in the stacked insulating film 402 may be formed by the same formation method. For example, both of them may be formed by an ALD method. Thus, they can be formed without exposure to the air. As a result, entry of impurities can be prevented. Furthermore, the formation can be performed in the same chamber. As a result, entry of impurities can be prevented. As described above, the same formation method may be employed in not only the case of the semiconductor 406 and the insulating film 402 but also the case of other layers which are adjacent to each other. Note that a method for manufacturing a semiconductor device of one embodiment of the present invention is not limited thereto.

Next, the conductive film 416 a and the conductive film 416 b are formed.

A conductive film to be the conductive film 416 a and the conductive film 416 b may be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

The conductive film 416 a and the conductive film 416 b are formed in such a manner that the conductive film to be the conductive film 416 a and the conductive film 416 b is formed and then partly etched. After that, a region of the semiconductor 406 not overlapping with the conductive film 416 a and the conductive film 416 b is etched. By using a condition where side surfaces of the semiconductor 406 can be etched, the semiconductor 406 can also be reduced in the channel width direction (see FIGS. 3A and 3B). It is preferable to employ a formation method by which the semiconductor 406 is not damaged when the conductive film is formed. In other words, the conductive film is preferably formed by an MCVD method or the like.

Note that in the case where each of the conductive films 416 a and 416 b is formed to have a stacked-layer structure, layers in the stacked-layer film may be formed by different formation methods such as a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, and an ALD method. For example, the first layer may be formed by an MOCVD method and the second layer may be formed by a sputtering method. Alternatively, the first layer may be formed by an ALD method and the second layer may be formed by an MOCVD method. Alternatively, the first layer may be formed by an ALD method and the second layer may be formed by a sputtering method. Alternatively, the first layer may be formed by an ALD method, the second layer may be formed by a sputtering method, and the third layer may be formed by an ALD method. When layers are formed by different formation methods as described above, the layers can have different functions or different properties. Furthermore, by stacking the layers, a more appropriate film can be formed as a stacked-layer film.

In other words, in the case where each of the conductive films 416 a and 416 b is formed to have a stacked-layer structure, for example, when an n-th layer is formed by at least one of a sputtering method, a CVD method (e.g., a PECVD method, a TCVD method, an MCVD method, or an MOCVD method), an MBE method, a PLD method, an ALD method, and the like and an n+1-th layer is formed by at least one of a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, an ALD method, and the like, the n-th layer and an n+1-th layer may be formed by different formation methods (n is a natural number). Note that the n-th layer and an n+2-th layer may be formed by the same formation method. Alternatively, all the layers may be formed by the same formation method.

Note that the conductive film 416 a (conductive film 416 b) or at least one layer in the stacked conductive film 416 a (conductive film 416 b) and the semiconductor 406 or at least one layer in the stacked semiconductor 406 may be formed by the same formation method. For example, both of them may be formed by an ALD method. Thus, they can be formed without exposure to the air. As a result, entry of impurities can be prevented. Alternatively, for example, the conductive film 416 a (conductive film 416 b) and the semiconductor 406 in contact with each other may be formed by the same formation method. Thus, the formation can be performed in the same chamber. As a result, entry of impurities can be prevented. As described above, the same formation method may be employed in not only the case of the semiconductor 406 and the conductive film 416 a (conductive film 416 b) but also the case of other layers which are adjacent to each other. Note that a method for manufacturing a semiconductor device of one embodiment of the present invention is not limited thereto.

Note that the conductive film 416 a (conductive film 416 b) or at least one layer in the stacked conductive film 416 a (conductive film 416 b), the semiconductor 406 or at least one layer in the stacked semiconductor 406, and the insulating film 402 or at least one layer in the stacked insulating film 402 may be formed by the same formation method. For example, all of them may be formed by an ALD method. Thus, they can be formed without exposure to the air. As a result, entry of impurities can be prevented. Note that a method for manufacturing a semiconductor device of one embodiment of the present invention is not limited thereto.

Next, the insulating film 412 is formed.

The insulating film 412 may be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

Note that in the case where the insulating film 412 is formed to have a stacked-layer structure, layers in the insulating film 412 may be formed by different formation methods such as a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, and an ALD method. For example, the first layer may be formed by an MOCVD method and the second layer may be formed by a sputtering method. Alternatively, the first layer may be formed by an ALD method and the second layer may be formed by an MOCVD method. Alternatively, the first layer may be formed by an ALD method and the second layer may be formed by a sputtering method. Alternatively, the first layer may be formed by an ALD method, the second layer may be formed by a sputtering method, and the third layer may be formed by an ALD method. When layers are formed by different formation methods as described above, the layers can have different functions or different properties. Furthermore, by stacking the layers, a more appropriate film can be formed as a stacked-layer film.

In other words, in the case where the insulating film 412 is a stacked-layer film, for example, an n-th layer is formed by at least one of a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, an ALD method, and the like and an n+1-th layer is formed by at least one of a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, an ALD method, and the like (n is a natural number). Note that the n-th layer and the n+1-th layer may be formed by different formation methods. Note that the n-th layer and the n+2-th layer may be formed by the same formation method. Alternatively, all the layers may be formed by the same formation method.

Note that the insulating film 412 or at least one layer in the stacked insulating film 412, and the conductive film 416 a (conductive film 416 b) or at least one layer in the stacked conductive film 416 a (conductive film 416 b) may be formed by the same formation method. For example, both of them may be formed by an ALD method. Thus, they can be formed without exposure to the air. As a result, entry of impurities can be prevented. Alternatively, for example, the conductive film 416 a (conductive film 416 b) and the insulating film 412 in contact with each other may be formed by the same formation method. Thus, the formation can be performed in the same chamber. As a result, entry of impurities can be prevented.

Note that the insulating film 412 or at least one layer in the stacked insulating film 412, the conductive film 416 a (conductive film 416 b) or at least one layer in the stacked conductive film 416 a (conductive film 416 b), the semiconductor 406 or at least one layer in the stacked semiconductor 406, and the insulating film 402 or at least one layer in the stacked insulating film 402 may be formed by the same formation method. For example, all of them may be formed by an ALD method. Thus, they can be formed without exposure to the air. As a result, entry of impurities can be prevented. Note that a method for manufacturing a semiconductor device of one embodiment of the present invention is not limited thereto.

Next, the conductive film 404 is formed (see FIGS. 4A and 4B).

A conductive film to be the conductive film 404 may be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

The insulating film 412 functions as a gate insulating film of a transistor. Therefore, the conductive film 404 is preferably formed by a formation method by which the insulating film 412 is not damaged when the conductive film to be the conductive film 404 is formed. In other words, the conductive film is preferably formed by an MCVD method or the like.

Note that in the case where the conductive film 404 is formed to have a stacked-layer structure, layers in the conductive film 404 may be formed by different formation methods such as a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, and an ALD method. For example, the first layer may be formed by an MOCVD method and the second layer may be formed by a sputtering method. Alternatively, the first layer may be formed by an ALD method and the second layer may be formed by an MOCVD method. Alternatively, the first layer may be formed by an ALD method and the second layer may be formed by a sputtering method. Alternatively, the first layer may be formed by an ALD method, the second layer may be formed by a sputtering method, and the third layer may be formed by an ALD method. Thus, when layers are formed by different formation methods, the layers can have different functions or different properties. Furthermore, by stacking the layers, a more appropriate film can be formed as a stacked-layer film.

In other words, in the case where the conductive film 404 is a stacked-layer film, for example, an n-th layer is formed by at least one of a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, an ALD method, and the like and an n+1-th layer is formed by at least one of a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, an ALD method, and the like (n is a natural number). Note that the n-th layer and the n+1-th layer may be formed by different formation methods. Note that the n-th layer and the n+2-th layer may be formed by the same formation method. Alternatively, all the layers may be formed by the same formation method.

Note that the conductive film 404 or at least one layer in the stacked conductive film 404, and the insulating film 412 or at least one layer in the stacked insulating film 412 may be formed by the same formation method. For example, both of them may be formed by an ALD method. Thus, they can be formed without exposure to the air. As a result, entry of impurities can be prevented. Alternatively, for example, the conductive film 404 and the insulating film 412 in contact with each other may be formed by the same formation method. Thus, the formation can be performed in the same chamber. As a result, entry of impurities can be prevented.

Note that the conductive film 404 or at least one layer in the stacked conductive film 404, the insulating film 412 or at least one layer in the stacked insulating film 412, the conductive film 416 a (conductive film 416 b) or at least one layer in the stacked conductive film 416 a (conductive film 416 b), the semiconductor 406 or at least one layer in the stacked semiconductor 406, and the insulating film 402 or at least one layer in the stacked insulating film 402 may be formed by the same formation method. For example, all of them may be formed by an ALD method. Thus, they can be formed without exposure to the air. As a result, entry of impurities can be prevented. Note that a method for manufacturing a semiconductor device of one embodiment of the present invention is not limited thereto.

Next, the insulating film 418 is formed.

The insulating film 418 may be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

Note that in the case where the insulating film 418 is formed to have a stacked-layer structure, layers in the insulating film 418 may be formed by different formation methods such as a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, and an ALD method. For example, the first layer may be formed by an MOCVD method and the second layer may be formed by a sputtering method. Alternatively, the first layer may be formed by an ALD method and the second layer may be formed by an MOCVD method. Alternatively, the first layer may be formed by an ALD method and the second layer may be formed by a sputtering method. Alternatively, the first layer may be formed by an ALD method, the second layer may be formed by a sputtering method, and the third layer may be formed by an ALD method. When layers are formed by different formation methods as described above, the layers can have different functions or different properties. Furthermore, by stacking the layers, a more appropriate film can be formed as a stacked-layer film.

In other words, in the case where the insulating film 418 is formed to have a stacked-layer structure, for example, when an n-th layer is formed by at least one of a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, an ALD method, and the like and an n+1-th layer is formed by at least one of a sputtering method, a CVD method (a PECVD method, a TCVD method, an MCVD method, an MOCVD method, or the like), an MBE method, a PLD method, an ALD method, and the like, the n-th layer and the n+1-th layer may be formed by different formation methods (n is a natural number). Note that the n-th layer and the n+2-th layer may be formed by the same formation method. Alternatively, all the layers may be formed by the same formation method.

Note that the insulating film 418 or at least one layer in the stacked insulating film 418, and the conductive film 416 a (conductive film 416 b) or at least one layer in the stacked conductive film 416 a (conductive film 416 b) may be formed by the same formation method. For example, both of them may be formed by an ALD method. Thus, they can be formed without exposure to the air. As a result, entry of impurities can be prevented. Alternatively, for example, the conductive film 416 a (conductive film 416 b) and the insulating film 418 in contact with each other may be formed by the same formation method. Thus, the formation can be performed in the same chamber. As a result, entry of impurities can be prevented.

Note that the insulating film 418 or at least one layer in the stacked insulating film 418, the conductive film 416 a (conductive film 416 b) or at least one layer in the stacked conductive film 416 a (conductive film 416 b), the semiconductor 406 or at least one layer in the stacked semiconductor 406, the insulating film 402 or at least one layer in the stacked insulating film 402, and the insulating film 412 or at least one layer in the stacked insulating film 412 may be formed by the same formation method. For example, all of them may be formed by an ALD method. Thus, they can be formed without exposure to the air. As a result, entry of impurities can be prevented. Note that a method for manufacturing a semiconductor device of one embodiment of the present invention is not limited thereto.

In this manner, a transistor of one embodiment of the present invention can be manufactured.

<Modification Example of Transistor Structure 1>

The top view may be similar to that of FIG. 1A as illustrated in FIG. 5A, and in the cross-sectional view illustrated in FIG. 5B, the oxide semiconductor layer 407 may be provided under the insulating film 412. As the oxide semiconductor layer 407, the semiconductor shown as the oxide semiconductor layer 406 c may be used. In that case, the semiconductor 406 may be a stacked-layer film illustrated in FIG. 5C. The description of FIG. 1C is referred to for the oxide semiconductor layer 406 a and the oxide semiconductor layer 406 b illustrated in FIG. 5C. Note that the description of the transistor in FIGS. 1A to 1C is referred to for the structures of the other components.

<Transistor Structure 2>

FIGS. 6A and 6B are a top view and a cross-sectional view of a transistor of one embodiment of the present invention. FIG. 6A is a top view and FIG. 6B is a cross-sectional view taken along dashed-dotted line B1-B2 and dashed-dotted line B3-B4 in FIG. 6A. Note that for simplification of the drawing, some components are not illustrated in the top view in FIG. 6A.

The transistor illustrated in FIGS. 6A and 6B includes a conductive film 614 over a substrate 600, an insulating film 602 over the conductive film 614, a semiconductor 606 over the insulating film 602, a conductive film 616 a and a conductive film 616 b in contact with a top surface and side surfaces of the semiconductor 606, an insulating film 612 over the semiconductor 606, the conductive film 616 a, and the conductive film 616 b, and a conductive film 604 in contact with a top surface of the insulating film 612 and a top surface and side surfaces of the semiconductor 606. Note that an insulating film may be provided between the substrate 600 and the conductive film 614. The conductive film 614 and/or the conductive film 604 function as a gate electrode of the transistor. Furthermore, the insulating film 602 and/or the insulating film 612 function as a gate insulating film of the transistor. Furthermore, the conductive film 616 a and the conductive film 616 b function as a source electrode and a drain electrode of the transistor.

Note that a protective insulating film covering the transistor may be provided. For the protective insulating film, the description of the insulating film 418 is referred to.

Note that the transistor may have an s-channel structure using the conductive film 604.

For the substrate 600, the description of the substrate 400 is referred to. For the conductive film 614, the description of the conductive film 404 is referred to. For the insulating film 602, the description of the insulating film 412 is referred to. For the semiconductor 606, the description of the semiconductor 406 is referred to. For the conductive film 616 a and the conductive film 616 b, the description of the conductive film 416 a and the conductive film 416 b is referred to. For the insulating film 612, the description of the insulating film 412 is referred to. For the conductive film 604, the description of the conductive film 404 is referred to.

<Semiconductor Device>

An example of a semiconductor device of one embodiment of the present invention is shown below.

<Circuit>

An example of a circuit including a transistor of one embodiment of the present invention is shown below.

[Cross-Sectional Structure]

FIG. 7A is a cross-sectional view of a semiconductor device of one embodiment of the present invention. The semiconductor device illustrated in FIG. 7A includes a transistor 2200 using a first semiconductor in a lower portion and a transistor 2100 using a second semiconductor in an upper portion. FIG. 7A shows an example in which the transistor illustrated in FIGS. 1A to 1C is used as the transistor 2100 using the second semiconductor.

As the first semiconductor, a semiconductor having an energy gap different from that of the second semiconductor may be used. For example, the first semiconductor may be a semiconductor other than an oxide semiconductor and the second semiconductor may be an oxide semiconductor. In the case where single crystal silicon is used as the first semiconductor, the transistor 2200 suitable for high speed operation can be obtained. Furthermore, in the case where an oxide semiconductor is used as the second semiconductor, the transistor 2100 suitable for low off-state current can be obtained.

Note that the transistor 2200 may be either an n-channel transistor or a p-channel transistor, and an appropriate transistor is used in accordance with a circuit. As the transistor 2100 and/or the transistor 2200, the above-described transistor or the transistor illustrated in FIG. 7A is not necessarily used in some cases.

The semiconductor device illustrated in FIG. 7A includes the transistor 2100 above the transistor 2200 with an insulating film 2201 and an insulating film 2207 provided therebetween. Between the transistor 2200 and the transistor 2100, a plurality of conductive films 2202 which function as wirings are provided. Wirings or electrodes provided in an upper layer and a lower layer are electrically connected to each other by a plurality of conductive films 2203 embedded in insulating films. Furthermore, the semiconductor device includes an insulating film 2204 over the transistor 2100, a conductive film 2205 over the insulating film 2204, and a conductive film 2206 formed in the same layer (through the same steps) as a source electrode and a drain electrode of the transistor 2100.

By stacking a plurality of transistors, a plurality of circuits can be arranged with high density.

Here, in the case where single crystal silicon is used as the first semiconductor of the transistor 2200, the hydrogen concentration in an insulating film near the first semiconductor of the transistor 2200 is preferably high. The hydrogen terminates dangling bonds of silicon, so that the reliability of the transistor 2200 can be increased. On the other hand, in the case where an oxide semiconductor is used as the second semiconductor of the transistor 2100, the hydrogen concentration in an insulating film near the second semiconductor of the transistor 2100 is preferably low. The hydrogen causes generation of carriers in the oxide semiconductor; thus, the reliability of the transistor 2100 might be decreased. Therefore, in the case where the transistor 2200 using single crystal silicon and the transistor 2100 using an oxide semiconductor are stacked, providing the insulating film 2207 having a function of blocking hydrogen between the transistors is effective because the reliability of the transistors can be increased.

The insulating film 2207 may be, for example, formed to have a single-layer structure or a stacked-layer structure using an insulating film containing aluminum oxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, hafnium oxynitride, yttria-stabilized zirconia (YSZ), or the like.

Furthermore, an insulating film having a function of blocking hydrogen is preferably formed over the transistor 2100 to cover the transistor 2100 using an oxide semiconductor. As the insulating film, an insulating film that is similar to the insulating film 2207 can be used, and in particular, an aluminum oxide film is preferably used. The aluminum oxide film has a high blocking effect of preventing penetration of both oxygen and impurities such as hydrogen and moisture. Thus, by using the aluminum oxide film as the insulating film covering the transistor 2100, release of oxygen from the oxide semiconductor included in the transistor 2100 can be prevented and entry of water and hydrogen into the oxide semiconductor can be prevented.

Note that the transistor 2200 can be a transistor of various types without being limited to a planar type transistor. For example, a FIN-type transistor can be used. An example of a cross-sectional view in this case is shown in FIG. 7D. An insulating layer 2212 is provided over a semiconductor substrate 2211. The semiconductor substrate 2211 includes a projection with a thin tip (also referred to a fin). Note that an insulating film may be provided over the projection. The insulating film serves as a mask used for forming the projection. Alternatively, the projection may not have the thin tip; a projection with a cuboid-like projection and a projection with a thick tip are permitted, for example. A gate insulating film 2214 is provided over the projection of the semiconductor substrate 2211, and a gate electrode 2213 is provided over the gate insulating film 2214. Source and drain regions 2215 are formed in the semiconductor substrate 2211. Note that here is shown an example in which the semiconductor substrate 2211 includes the projection; however, a semiconductor device of one embodiment of the present invention is not limited thereto. For example, a semiconductor region having a projection may be formed by processing an SOI substrate.

[Circuit Configuration Example]

In the above circuit, electrodes of the transistor 2100 and the transistor 2200 can be connected in a variety of ways; thus, a variety of circuits can be formed. Examples of circuit configurations which can be achieved by using a semiconductor device of one embodiment of the present invention are shown below.

[CMOS Inverter]

A circuit diagram in FIG. 7B shows a configuration of a so-called CMOS inverter in which the p-channel transistor 2200 and the n-channel transistor 2100 are connected to each other in series and in which gates of them are connected to each other.

[CMOS Analog Switch]

A circuit diagram in FIG. 7C shows a configuration in which sources of the transistors 2100 and 2200 are connected to each other and drains of the transistors 2100 and 2200 are connected to each other. With such a configuration, the transistors can function as a so-called CMOS analog switch.

[Memory Device Example]

An example of a semiconductor device (memory device) which includes the transistor of one embodiment of the present invention, which can retain stored data even when not powered, and which has an unlimited number of write cycles is shown in FIGS. 8A and 8B.

The semiconductor device illustrated in FIG. 8A includes a transistor 3200 using a first semiconductor, a transistor 3300 using a second semiconductor, and a capacitor 3400. Note that any of the above-described transistors can be used as the transistor 3300.

The transistor 3300 is a transistor using an oxide semiconductor. Since the off-state current of the transistor 3300 is low, stored data can be retained for a long period at a predetermined node of the semiconductor device. In other words, power consumption of the semiconductor device can be reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low.

In FIG. 8A, a first wiring 3001 is electrically connected to a source of the transistor 3200. A second wiring 3002 is electrically connected to a drain of the transistor 3200. A third wiring 3003 is electrically connected to one of the source and the drain of the transistor 3300. A fourth wiring 3004 is electrically connected to the gate of the transistor 3300. The gate of the transistor 3200 and the other of the source and the drain of the transistor 3300 are electrically connected to the one electrode of the capacitor 3400. A fifth wiring 3005 is electrically connected to the other electrode of the capacitor 3400.

The semiconductor device in FIG. 8A has a feature that the potential of the gate of the transistor 3200 can be retained, and thus enables writing, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of the fourth wiring 3004 is set to a potential at which the transistor 3300 is turned on, so that the transistor 3300 is turned on. Accordingly, the potential of the third wiring 3003 is supplied to a node FG where the gate of the transistor 3200 and the one electrode of the capacitor 3400 are electrically connected to each other. That is, a predetermined charge is supplied to the gate of the transistor 3200 (writing). Here, one of two kinds of charges providing different potential levels (hereinafter referred to as a low-level charge and a high-level charge) is supplied. After that, the potential of the fourth wiring 3004 is set to a potential at which the transistor 3300 is turned off, so that the transistor 3300 is turned off Thus, the charge is held at the node FG (retaining).

Since the off-state current of the transistor 3300 is extremely low, the charge of the node FG is retained for a long time.

Next, reading of data is described. An appropriate potential (a reading potential) is supplied to the fifth wiring 3005 while a predetermined potential (a constant potential) is supplied to the first wiring 3001, whereby the potential of the second wiring 3002 varies depending on the amount of charge retained in the node FG. This is because in the case of using an n-channel transistor as the transistor 3200, an apparent threshold voltage V_(th) _(_) _(H) at the time when the high-level charge is given to the gate of the transistor 3200 is lower than an apparent threshold voltage V_(th) _(_) _(L) at the time when the low-level charge is given to the gate of the transistor 3200. Here, an apparent threshold voltage refers to the potential of the fifth wiring 3005 which is needed to turn on the transistor 3200. Thus, the potential of the fifth wiring 3005 is set to a potential V₀ which is between V_(th) _(_) _(H) and V_(th) _(_) _(L), whereby charge supplied to the node FG can be determined. For example, in the case where the high-level charge is supplied to the node FG in writing and the potential of the fifth wiring 3005 is V₀ (>V_(th) _(_) _(H)), the transistor 3200 is turned on. On the other hand, in the case where the low-level charge is supplied to the node FG, even when the potential of the fifth wiring 3005 is V₀ (<V_(th) _(_) _(L)), the transistor 3200 remains off Thus, the data retained in the node FG can be read by determining the potential of the second wiring 3002.

Note that in the case where memory cells are arrayed, it is necessary that data of a desired memory cell is read in read operation. In the case where data of the other memory cells is not read, the fifth wiring 3005 may be supplied with a potential at which the transistor 3200 is turned off regardless of the charge supplied to the node FG, that is, a potential lower than V_(th) _(_) _(H). Alternatively, the fifth wiring 3005 may be supplied with a potential at which the transistor 3200 is turned on regardless of the charge supplied to the node FG, that is, a potential higher than V_(th) _(_) _(L).

The semiconductor device in FIG. 8B is different form the semiconductor device in FIG. 8A in that the transistor 3200 is not provided. Also in this case, writing and retaining operation of data can be performed in a manner similar to the semiconductor device in FIG. 8A.

Reading of data in the semiconductor device in FIG. 8B is described. When the transistor 3300 is turned on, the third wiring 3003 which is in a floating state and the capacitor 3400 are electrically connected to each other, and the charge is redistributed between the third wiring 3003 and the capacitor 3400. As a result, the potential of the third wiring 3003 is changed. The amount of change in potential of the third wiring 3003 varies depending on the potential of the one electrode of the capacitor 3400 (or the charge accumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the charge redistribution is (C_(B)×V_(B0)+C×V)/(G_(B)+C), where V is the potential of the one electrode of the capacitor 3400, C is the capacitance of the capacitor 3400, G_(B) is the capacitance component of the third wiring 3003, and V_(B0) is the potential of the third wiring 3003 before the charge redistribution. Thus, it can be found that, assuming that the memory cell is in either of two states in which the potential of the one electrode of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potential of the third wiring 3003 in the case of retaining the potential V₁ (=(C_(B)×V_(B0)+C×V)/(G_(B)+C)) is higher than the potential of the third wiring 3003 in the case of retaining the potential V₀ (=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with a predetermined potential, data can be read.

In this case, a transistor including the first semiconductor may be used for a driver circuit for driving a memory cell, and a transistor including the second semiconductor may be stacked over the driver circuit as the transistor 3300.

When including a transistor using an oxide semiconductor and having an extremely low off-state current, the semiconductor device described above can retain stored data for a long time. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely low, which leads to a sufficient reduction in power consumption. Moreover, stored data can be retained for a long time even when power is not supplied (note that a potential is preferably fixed).

Furthermore, in the semiconductor device, high voltage is not needed for writing data and deterioration of elements is less likely to occur. Unlike in a conventional nonvolatile memory, for example, it is not necessary to inject and extract electrons into and from a floating gate; thus, a problem such as deterioration of an insulating film is not caused. That is, the semiconductor device of one embodiment of the present invention does not have a limit on the number of times data can be rewritten, which is a problem of a conventional nonvolatile memory, and the reliability thereof is drastically improved. Furthermore, data is written depending on the state of the transistor (on or off), whereby high-speed operation can be easily achieved.

<RFID Tag>

An RFID tag including the transistor or the memory device is described below with reference to FIG. 9.

The RFID tag of one embodiment of the present invention includes a memory circuit, stores data in the memory circuit, and transmits and receives data to/from the outside by using contactless means, for example, wireless communication. With these features, the RFID tag can be used for an individual authentication system in which an object or the like is recognized by reading the individual information, for example. Note that the RFID tag is required to have high reliability in order to be used for this purpose.

A configuration of the RFID tag will be described with reference to FIG. 9. FIG. 9 is a block diagram illustrating a configuration example of an RFID tag.

As shown in FIG. 9, an RFID tag 800 includes an antenna 804 which receives a radio signal 803 that is transmitted from an antenna 802 connected to a communication device 801 (also referred to as an interrogator, a reader/writer, or the like). The RFID tag 800 includes a rectifier circuit 805, a constant voltage circuit 806, a demodulation circuit 807, a modulation circuit 808, a logic circuit 809, a memory circuit 810, and a ROM 811. A semiconductor of a transistor having a rectifying function included in the demodulation circuit 807 may be a material which enables a reverse current to be low enough, for example, an oxide semiconductor. This can suppress the phenomenon of a rectifying function becoming weaker due to generation of a reverse current and prevent saturation of the output from the demodulation circuit. In other words, the input to the demodulation circuit and the output from the demodulation circuit can have a relation closer to a linear relation. Note that data transmission methods are roughly classified into the following three methods: an electromagnetic coupling method in which a pair of coils is provided so as to face each other and communicates with each other by mutual induction, an electromagnetic induction method in which communication is performed using an induction field, and a radio wave method in which communication is performed using a radio wave. Any of these methods can be used in the RFID tag 800.

Next, the structure of each circuit will be described. The antenna 804 exchanges the radio signal 803 with the antenna 802 which is connected to the communication device 801. The rectifier circuit 805 generates an input potential by rectification, for example, half-wave voltage doubler rectification of an input alternating signal generated by reception of a radio signal at the antenna 804 and smoothing of the rectified signal with a capacitor provided in a later stage in the rectifier circuit 805. Note that a limiter circuit may be provided on an input side or an output side of the rectifier circuit 805. The limiter circuit controls electric power so that electric power which is higher than or equal to certain electric power is not input to a circuit in a later stage if the amplitude of the input alternating signal is high and an internal generation voltage is high.

The constant voltage circuit 806 generates a stable power supply voltage from an input potential and supplies it to each circuit. Note that the constant voltage circuit 806 may include a reset signal generation circuit. The reset signal generation circuit is a circuit which generates a reset signal of the logic circuit 809 by utilizing rise of the stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal by envelope detection and generates the demodulated signal. Furthermore, the modulation circuit 808 performs modulation in accordance with data to be output from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. The memory circuit 810 holds the input data and includes a row decoder, a column decoder, a memory region, and the like. Furthermore, the ROM 811 stores an identification number (ID) or the like and outputs it in accordance with processing.

Note that the decision whether each circuit described above is provided or not can be made as appropriate.

Here, the above-described memory device can be used as the memory circuit 810. Since the memory device of one embodiment of the present invention can retain data even when not powered, the memory device is suitable for an RFID tag. Furthermore, the memory device of one embodiment of the present invention needs power (voltage) needed for data writing lower than that needed in a conventional nonvolatile memory; thus, it is possible to prevent a difference between the maximum communication range in data reading and that in data writing. In addition, it is possible to suppress malfunction or incorrect writing which is caused by power shortage in data writing.

Since the memory device of one embodiment of the present invention can be used as a nonvolatile memory, it can also be used as the ROM 811. In this case, it is preferable that a manufacturer separately prepare a command for writing data to the ROM 811 so that a user cannot rewrite data freely. Since the manufacturer gives identification numbers before shipment and then starts shipment of products, instead of putting identification numbers to all the manufactured RFID tags, it is possible to put identification numbers to only good products to be shipped. Thus, the identification numbers of the shipped products are in series and customer management corresponding to the shipped products is easily performed.

<Application Examples of RFID Tag>

Application examples of the RFID tag of one embodiment of the present invention are shown below with reference to FIGS. 10A to 10F. The RFID tag is widely used and can be provided for, for example, products such as bills, coins, securities, bearer bonds, documents (e.g., driver's licenses or resident's cards, see FIG. 10A), packaging containers (e.g., wrapping paper or bottles, see FIG. 10C), recording media (e.g., DVD or video tapes, see FIG. 10B), vehicles (e.g., bicycles, see FIG. 10D), personal belongings (e.g., bags or glasses), foods, plants, animals, human bodies, clothing, household goods, medical supplies such as medicine and chemicals, and electronic devices (e.g., liquid crystal display devices, EL display devices, television sets, or cellular phones), or tags on products (see FIGS. 10E and 10F).

An RFID tag 4000 of one embodiment of the present invention is fixed to products by being attached to a surface thereof or embedded therein. For example, the RFID tag 4000 is fixed to each product by being embedded in paper of a book, or embedded in an organic resin of a package. The RFID tag 4000 of one embodiment of the present invention is small, thin, and lightweight, so that the design of a product is not impaired even after the RFID tag 4000 of one embodiment of the present invention is fixed thereto. Furthermore, bills, coins, securities, bearer bonds, documents, or the like can have identification functions by being provided with the RFID tag 4000 of one embodiment of the present invention, and the identification functions can be utilized to prevent counterfeits. Moreover, the efficiency of a system such as an inspection system can be improved by providing the RFID tag 4000 of one embodiment of the present invention for packaging containers, recording media, personal belongings, foods, clothing, household goods, electronic devices, or the like. Vehicles can also have higher security against theft or the like by being provided with the RFID tag 4000 of one embodiment of the present invention.

As described above, the RFID tag of one embodiment of the present invention can be used for the above-described purposes.

<CPU>

A CPU including a semiconductor device such as any of the above-described transistors or the above-described memory device is described below.

FIG. 11 is a block diagram illustrating a configuration example of a CPU including any of the above-described transistors as a component.

The CPU illustrated in FIG. 11 includes, over a substrate 1190, an arithmetic logic unit (ALU) 1191, an ALU controller 1192, an instruction decoder 1193, an interrupt controller 1194, a timing controller 1195, a register 1196, a register controller 1197, a bus interface 1198 (Bus I/F), a rewritable ROM 1199, and a ROM interface 1189 (ROM I/F). A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate 1190. The ROM 1199 and the ROM interface 1189 may be provided over a separate chip. Needless to say, the CPU in FIG. 11 is just an example in which the configuration has been simplified, and an actual CPU may have a variety of configurations depending on the application. For example, the CPU may have the following configuration: a structure including the CPU illustrated in FIG. 11 or an arithmetic circuit is considered as one core; a plurality of the cores are included; and the cores operate in parallel. The number of bits that the CPU can process in an internal arithmetic circuit or in a data bus can be 8, 16, 32, or 64, for example.

An instruction that is input to the CPU through the bus interface 1198 is input to the instruction decoder 1193 and decoded therein, and then, input to the ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195 conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller 1192 generates signals for controlling the operation of the ALU 1191. While the CPU is executing a program, the interrupt controller 1194 judges an interrupt request from an external input/output device or a peripheral circuit on the basis of its priority or a mask state, and processes the request. The register controller 1197 generates an address of the register 1196, and reads/writes data from/to the register 1196 in accordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operation timings of the ALU 1191, the ALU controller 1192, the instruction decoder 1193, the interrupt controller 1194, and the register controller 1197. For example, the timing controller 1195 includes an internal clock generator for generating an internal clock signal CLK2 based on a reference clock signal CLK1, and supplies the internal clock signal CLK2 to the above circuits.

In the CPU illustrated in FIG. 11, a memory cell is provided in the register 1196. For the memory cell of the register 1196, any of the above-described transistors, the above-described memory device, or the like can be used.

In the CPU illustrated in FIG. 11, the register controller 1197 selects operation of retaining data in the register 1196 in accordance with an instruction from the ALU 1191. That is, the register controller 1197 selects whether data is retained by a flip-flop or by a capacitor in the memory cell included in the register 1196. When data retaining by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register 1196. When data retaining by the capacitor is selected, the data is rewritten in the capacitor, and supply of power supply voltage to the memory cell in the register 1196 can be stopped.

FIG. 12 is an example of a circuit diagram of a memory element that can be used as the register 1196. A memory element 1200 includes a circuit 1201 in which stored data is volatile when power supply is stopped, a circuit 1202 in which stored data is nonvolatile even when power supply is stopped, a switch 1203, a switch 1204, a logic element 1206, a capacitor 1207, and a circuit 1220 having a selecting function. The circuit 1202 includes a capacitor 1208, a transistor 1209, and a transistor 1210. Note that the memory element 1200 may further include another element such as a diode, a resistor, or an inductor, as needed.

Here, the above-described memory device can be used as the circuit 1202. When supply of a power supply voltage to the memory element 1200 is stopped, GND (0 V) or a potential at which the transistor 1209 in the circuit 1202 is turned off continues to be input to a gate of the transistor 1209. For example, the gate of the transistor 1209 is grounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213 having one conductivity type (e.g., an n-channel transistor) and the switch 1204 is a transistor 1214 having a conductivity type opposite to the one conductivity type (e.g., a p-channel transistor). A first terminal of the switch 1203 corresponds to one of a source and a drain of the transistor 1213, a second terminal of the switch 1203 corresponds to the other of the source and the drain of the transistor 1213, and conduction or non-conduction between the first terminal and the second terminal of the switch 1203 (i.e., the on/off state of the transistor 1213) is selected by a control signal RD input to a gate of the transistor 1213. A first terminal of the switch 1204 corresponds to one of a source and a drain of the transistor 1214, a second terminal of the switch 1204 corresponds to the other of the source and the drain of the transistor 1214, and conduction or non-conduction between the first terminal and the second terminal of the switch 1204 (i.e., the on/off state of the transistor 1214) is selected by the control signal RD input to a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electrically connected to one of a pair of electrodes of the capacitor 1208 and a gate of the transistor 1210. Here, the connection portion is referred to as a node M2. One of a source and a drain of the transistor 1210 is electrically connected to a line which can supply a low power supply potential (e.g., a GND line), and the other thereof is electrically connected to the first terminal of the switch 1203 (the one of the source and the drain of the transistor 1213). The second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is electrically connected to the first terminal of the switch 1204 (the one of the source and the drain of the transistor 1214). The second terminal of the switch 1204 (the other of the source and the drain of the transistor 1214) is electrically connected to a line which can supply a power supply potential VDD. The second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213), the first terminal of the switch 1204 (the one of the source and the drain of the transistor 1214), an input terminal of the logic element 1206, and one of a pair of electrodes of the capacitor 1207 are electrically connected to each other. Here, the connection portion is referred to as a node M1. The other of the pair of electrodes of the capacitor 1207 can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor 1207 can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor 1207 is electrically connected to the line which can supply a low power supply potential (e.g., a GND line). The other of the pair of electrodes of the capacitor 1208 can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor 1208 can be supplied with the low power supply potential (e.g., GND) or the high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor 1208 is electrically connected to the line which can supply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily provided as long as the parasitic capacitance of the transistor, the wiring, or the like is actively utilized.

A control signal WE is input to the gate of the transistor 1209. As for each of the switch 1203 and the switch 1204, a conduction state or a non-conduction state between the first terminal and the second terminal is selected by the control signal RD which is different from the control signal WE. When the first terminal and the second terminal of one of the switches are in the conduction state, the first terminal and the second terminal of the other of the switches are in the non-conduction state.

A signal corresponding to data retained in the circuit 1201 is input to the other of the source and the drain of the transistor 1209. FIG. 12 illustrates an example in which a signal output from the circuit 1201 is input to the other of the source and the drain of the transistor 1209. The logic value of a signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is inverted by the logic element 1206, and the inverted signal is input to the circuit 1201 through the circuit 1220.

In the example of FIG. 12, a signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is input to the circuit 1201 through the logic element 1206 and the circuit 1220; however, one embodiment of the present invention is not limited thereto. The signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) may be input to the circuit 1201 without its logic value being inverted. For example, in the case where the circuit 1201 includes a node in which a signal obtained by inversion of the logic value of a signal input from the input terminal is retained, the signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) can be input to the node.

In FIG. 12, the transistors included in the memory element 1200 except for the transistor 1209 can each be a transistor in which a channel is formed in a semiconductor other than an oxide semiconductor or in the substrate 1190. For example, the transistor can be a transistor whose channel is formed in a silicon film or a silicon substrate. Alternatively, all the transistors in the memory element 1200 may be a transistor in which a channel is formed in an oxide semiconductor. Further alternatively, in the memory element 1200, a transistor in which a channel is formed in an oxide semiconductor can be included besides the transistor 1209, and a transistor in which a channel is formed in a layer or the substrate 1190 including a semiconductor other than an oxide semiconductor can be used for the rest of the transistors.

As the circuit 1201 in FIG. 12, for example, a flip-flop circuit can be used. As the logic element 1206, for example, an inverter or a clocked inverter can be used.

In a period during which the memory element 1200 is not supplied with the power supply voltage, the semiconductor device of one embodiment of the present invention can retain data stored in the circuit 1201 by the capacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in an oxide semiconductor is extremely low. For example, the off-state current of a transistor in which a channel is formed in an oxide semiconductor is significantly lower than that of a transistor in which a channel is formed in silicon having crystallinity. Thus, when the transistor is used as the transistor 1209, a signal held in the capacitor 1208 is retained for a long time also in a period during which the power supply voltage is not supplied to the memory element 1200. The memory element 1200 can accordingly retain the stored content (data) also in a period during which the supply of the power supply voltage is stopped.

Since the memory element performs pre-charge operation with the switch 1203 and the switch 1204, the time required for the circuit 1201 to retain original data again after the supply of the power supply voltage is restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input to the gate of the transistor 1210. Therefore, after supply of the power supply voltage to the memory element 1200 is restarted, the signal retained by the capacitor 1208 can be converted into the one corresponding to the state (the on state or the off state) of the transistor 1210 to be read from the circuit 1202. Consequently, an original signal can be accurately read even when a potential corresponding to the signal retained by the capacitor 1208 varies to some degree.

By applying the above-described memory element 1200 to a memory device such as a register or a cache memory included in a processor, data in the memory device can be prevented from being lost owing to the stop of the supply of the power supply voltage. Furthermore, shortly after the supply of the power supply voltage is restarted, the memory device can be returned to the same state as that before the power supply is stopped. Therefore, the power supply can be stopped even for a short time in the processor or one or a plurality of logic circuits included in the processor, resulting in lower power consumption.

Although the memory element 1200 is used in a CPU, the memory element 1200 can also be used in an LSI such as a digital signal processor (DSP), a custom LSI, or a programmable logic device (PLD), and a radio frequency identification (RF-ID).

<Display Device>

The following shows configuration examples of a display device of one embodiment of the present invention.

[Configuration Example]

FIG. 13A is a top view of a display device of one embodiment of the present invention. FIG. 13B illustrates a pixel circuit where a liquid crystal element is used for a pixel of a display device of one embodiment of the present invention. FIG. 13C illustrates a pixel circuit where an organic EL element is used for a pixel of a display device of one embodiment of the present invention.

Any of the above-described transistors can be used as a transistor used for the pixel. Here, an example in which an n-channel transistor is used is shown. Note that a transistor manufactured through the same steps as the transistor used for the pixel may be used for a driver circuit. Thus, by using any of the above-described transistors for a pixel or a driver circuit, the display device can have high display quality and/or high reliability.

FIG. 13A illustrates an example of an active matrix display device. A pixel portion 5001, a first scan line driver circuit 5002, a second scan line driver circuit 5003, and a signal line driver circuit 5004 are provided over a substrate 5000 in the display device. The pixel portion 5001 is electrically connected to the signal line driver circuit 5004 through a plurality of signal lines and is electrically connected to the first scan line driver circuit 5002 and the second scan line driver circuit 5003 through a plurality of scan lines. Pixels including display elements are provided in respective regions divided by the scan lines and the signal lines. The substrate 5000 of the display device is electrically connected to a timing control circuit (also referred to as a controller or a control IC) through a connection portion such as a flexible printed circuit (FPC).

The first scan line driver circuit 5002, the second scan line driver circuit 5003, and the signal line driver circuit 5004 are formed over the substrate 5000 where the pixel portion 5001 is formed. Therefore, a display device can be manufactured at cost lower than that in the case where a driver circuit is separately formed. Furthermore, in the case where a driver circuit is separately formed, the number of wiring connections is increased. By providing the driver circuit over the substrate 5000, the number of wiring connections can be reduced. Accordingly, the reliability and/or yield can be improved.

[Liquid Crystal Display Device]

FIG. 13B illustrates an example of a circuit configuration of the pixel. Here, a pixel circuit which is applicable to a pixel of a VA liquid crystal display device, or the like is illustrated.

This pixel circuit can be applied to a structure in which one pixel includes a plurality of pixel electrodes. The pixel electrodes are connected to different transistors, and the transistors can be driven with different gate signals. Accordingly, signals applied to individual pixel electrodes in a multi-domain pixel can be controlled independently.

A gate wiring 5012 of a transistor 5016 and a gate wiring 5013 of a transistor 5017 are separated so that different gate signals can be supplied thereto. In contrast, a source or drain electrode 5014 functioning as a data line is shared by the transistors 5016 and 5017. Any of the above-described transistors can be used as appropriate as each of the transistors 5016 and 5017. Thus, a liquid crystal display device having high display quality and/or high reliability can be provided.

The shapes of a first pixel electrode electrically connected to the transistor 5016 and a second pixel electrode electrically connected to the transistor 5017 are described. The first pixel electrode and the second pixel electrode are separated by a slit. The first pixel electrode has a V shape and the second pixel electrode is provided so as to surround the first pixel electrode.

A gate electrode of the transistor 5016 is electrically connected to the gate wiring 5012, and a gate electrode of the transistor 5017 is electrically connected to the gate wiring 5013. When different gate signals are supplied to the gate wiring 5012 and the gate wiring 5013, operation timings of the transistor 5016 and the transistor 5017 can be varied. As a result, alignment of liquid crystals can be controlled.

Furthermore, a capacitor may be formed using a capacitor wiring 5010, a gate insulating film functioning as a dielectric, and a capacitor electrode electrically connected to the first pixel electrode or the second pixel electrode.

The multi-domain pixel includes a first liquid crystal element 5018 and a second liquid crystal element 5019. The first liquid crystal element 5018 includes the first pixel electrode, a counter electrode, and a liquid crystal layer therebetween. The second liquid crystal element 5019 includes the second pixel electrode, a counter electrode, and a liquid crystal layer therebetween.

Note that a pixel circuit in the display device of one embodiment of the present invention is not limited to that shown in FIG. 13B. For example, a switch, a resistor, a capacitor, a transistor, a sensor, a logic circuit, or the like may be added to the pixel circuit shown in FIG. 13B.

[Organic EL Panel]

FIG. 13C illustrates another example of a circuit configuration of the pixel. Here, a pixel structure of a display device using an organic EL element is shown.

In an organic EL element, by application of voltage to a light-emitting element, electrons are injected from one of a pair of electrodes included in the organic EL element and holes are injected from the other of the pair of electrodes, into a layer containing a light-emitting organic compound; thus, current flows. The electrons and holes are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Owing to such a mechanism, this light-emitting element is referred to as a current-excitation light-emitting element.

FIG. 13C illustrates an example of a pixel circuit. Here, one pixel includes two n-channel transistors. Note that any of the above-described transistors can be used as the n-channel transistors. Furthermore, digital time grayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of a pixel employing digital time grayscale driving will be described.

A pixel 5020 includes a switching transistor 5021, a driver transistor 5022, a light-emitting element 5024, and a capacitor 5023. A gate electrode of the switching transistor 5021 is connected to a scan line 5026, a first electrode (one of a source electrode and a drain electrode) of the switching transistor 5021 is connected to a signal line 5025, and a second electrode (the other of the source electrode and the drain electrode) of the switching transistor 5021 is connected to a gate electrode of the driver transistor 5022. The gate electrode of the driver transistor 5022 is connected to a power supply line 5027 through the capacitor 5023, a first electrode of the driver transistor 5022 is connected to the power supply line 5027, and a second electrode of the driver transistor 5022 is connected to a first electrode (a pixel electrode) of the light-emitting element 5024. A second electrode of the light-emitting element 5024 corresponds to a common electrode 5028. The common electrode 5028 is electrically connected to a common potential line provided over the same substrate.

As each of the switching transistor 5021 and the driver transistor 5022, any of the above-described transistors can be used as appropriate. In this manner, an organic EL display device having high display quality and/or high reliability can be provided.

The potential of the second electrode (the common electrode 5028) of the light-emitting element 5024 is set to be a low power supply potential. Note that the low power supply potential is lower than a high power supply potential supplied to the power supply line 5027. For example, the low power supply potential can be GND, 0 V, or the like. The high power supply potential and the low power supply potential are set to be higher than or equal to the forward threshold voltage of the light-emitting element 5024, and the difference between the potentials is applied to the light-emitting element 5024, whereby current is supplied to the light-emitting element 5024, leading to light emission. The forward voltage of the light-emitting element 5024 refers to a voltage at which a desired luminance is obtained, and includes at least forward threshold voltage.

Note that gate capacitance of the driver transistor 5022 may be used as a substitute for the capacitor 5023 in some cases, so that the capacitor 5023 can be omitted. The gate capacitance of the driver transistor 5022 may be formed between the channel formation region and the gate electrode.

Next, a signal input to the driver transistor 5022 is described. In the case of a voltage-input voltage driving method, a video signal for turning on or off the driver transistor 5022 is input to the driver transistor 5022. In order for the driver transistor 5022 to operate in a linear region, voltage higher than the voltage of the power supply line 5027 is applied to the gate electrode of the driver transistor 5022. Note that voltage higher than or equal to voltage which is the sum of power supply line voltage and the threshold voltage V_(th) of the driver transistor 5022 is applied to the signal line 5025.

In the case of performing analog grayscale driving, a voltage higher than or equal to a voltage which is the sum of the forward voltage of the light-emitting element 5024 and the threshold voltage V_(th) of the driver transistor 5022 is applied to the gate electrode of the driver transistor 5022. A video signal by which the driver transistor 5022 is operated in a saturation region is input, so that current is supplied to the light-emitting element 5024. In order for the driver transistor 5022 to operate in a saturation region, the potential of the power supply line 5027 is set higher than the gate potential of the driver transistor 5022. When an analog video signal is used, it is possible to supply current to the light-emitting element 5024 in accordance with the video signal and perform analog grayscale driving.

Note that in the display device of one embodiment of the present invention, a pixel configuration is not limited to that shown in FIG. 13C. For example, a switch, a resistor, a capacitor, a sensor, a transistor, a logic circuit, or the like may be added to the pixel circuit shown in FIG. 13C.

In the case where any of the above-described transistors is used for the circuit shown in FIGS. 13A to 13C, the source electrode (the first electrode) is electrically connected to the low potential side and the drain electrode (the second electrode) is electrically connected to the high potential side. Furthermore, the potential of the first gate electrode may be controlled by a control circuit or the like and the potential described above as an example, e.g., a potential lower than the potential applied to the source electrode, may be input to the second gate electrode.

<Module>

A display module using a semiconductor device of one embodiment of the present invention is described below with reference to FIG. 14.

In a display module 8000 in FIG. 14, a touch panel 8004 connected to an FPC 8003, a cell 8006 connected to an FPC 8005, a backlight unit 8007, a frame 8009, a printed board 8010, and a battery 8011 are provided between an upper cover 8001 and a lower cover 8002. Note that the backlight unit 8007, the battery 8011, the touch panel 8004, and the like are not provided in some cases.

The semiconductor device of one embodiment of the present invention can be used for the cell 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002 can be changed as appropriate in accordance with the sizes of the touch panel 8004 and the cell 8006.

The touch panel 8004 can be a resistive touch panel or a capacitive touch panel and may be formed to overlap with the cell 8006. A counter substrate (sealing substrate) of the cell 8006 can have a touch panel function. A photosensor may be provided in each pixel of the cell 8006 so that an optical touch panel is obtained. An electrode for a touch sensor may be provided in each pixel of the cell 8006 so that a capacitive touch panel is obtained.

The backlight unit 8007 includes a light source 8008. The light source 8008 may be provided at an end portion of the backlight unit 8007 and a light diffusing plate may be used.

The frame 8009 may protect the cell 8006 and also function as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board 8010. The frame 8009 may function as a radiator plate.

The printed board 8010 has a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or a power source using the battery 8011 provided separately may be used. The battery 8011 can be omitted in the case of using a commercial power source.

The display module 8000 can be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet

<Electronic Device>

The semiconductor device of one embodiment of the present invention can be used for display devices, personal computers, or image reproducing devices provided with recording media (typically, devices which reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images). Other examples of electronic devices that can be equipped with the semiconductor device of one embodiment of the present invention are mobile phones, game machines including portable game consoles, portable data appliances, e-book readers, cameras such as video cameras and digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), and vending machines. FIGS. 15A to 15F illustrate specific examples of these electronic devices.

FIG. 15A illustrates a portable game console including a housing 901, a housing 902, a display portion 903, a display portion 904, a microphone 905, a speaker 906, an operation key 907, a stylus 908, and the like. Although the portable game machine in FIG. 15A has the two display portions 903 and 904, the number of display portions included in a portable game machine is not limited to this.

FIG. 15B illustrates a portable data terminal including a first housing 911, a second housing 912, a first display portion 913, a second display portion 914, a joint 915, an operation key 916, and the like. The first display portion 913 is provided in the first housing 911, and the second display portion 914 is provided in the second housing 912. The first housing 911 and the second housing 912 are connected to each other with the joint 915, and the angle between the first housing 911 and the second housing 912 can be changed with the joint 915. An image on the first display portion 913 may be switched depending on the angle between the first housing 911 and the second housing 912 at the joint 915. A display device with a position input function may be used as at least one of the first display portion 913 and the second display portion 914. Note that the position input function can be added by providing a touch panel in a display device. Alternatively, the position input function can be added by provision of a photoelectric conversion element called a photosensor in a pixel portion of a display device.

FIG. 15C illustrates a laptop personal computer including a housing 921, a display portion 922, a keyboard 923, a pointing device 924, and the like.

FIG. 15D illustrates an electric refrigerator-freezer including a housing 931, a door for a refrigerator 932, a door for a freezer 933, and the like.

FIG. 15E illustrates a video camera including a first housing 941, a second housing 942, a display portion 943, operation keys 944, a lens 945, a joint 946, and the like. The operation keys 944 and the lens 945 are provided for the first housing 941, and the display portion 943 is provided for the second housing 942. The first housing 941 and the second housing 942 are connected to each other with the joint 946, and the angle between the first housing 941 and the second housing 942 can be changed with the joint 946. Images displayed on the display portion 943 may be switched in accordance with the angle at the joint 946 between the first housing 941 and the second housing 942.

FIG. 15F illustrates an ordinary vehicle including a car body 951, wheels 952, a dashboard 953, lights 954, and the like.

EXAMPLE 1

In this example, differences in electrical characteristics depending on the shapes of semiconductors of transistors were calculated by device simulation.

Sentaurus manufactured by Synopsys, Inc. was used for the calculation, and a 3D structure was used. FIGS. 16A and 16B illustrate the structure of a transistor used for the calculation. FIG. 16A is a top view of the transistor. FIG. 16B is a cross-sectional view taken along dashed-dotted line F1-F2 and dashed-dotted line F3-F4 in FIG. 16A.

The transistor illustrated in FIG. 16B includes an insulating film 102; an oxide semiconductor layer 106 a over the insulating film 102; an oxide semiconductor layer 106 b having a projection and a depression over the oxide semiconductor layer 106 a; a conductive film 116 a and a conductive film 116 b in contact with a top surface and side surfaces of the projection of the oxide semiconductor layer 106 b, side surfaces of the oxide semiconductor layer 106 a, and side surfaces of the insulating film 102; an oxide semiconductor layer 107 in contact with a top surface and side surfaces of the depression of the oxide semiconductor layer 106 b, the side surfaces of the oxide semiconductor layer 106 a, and the side surfaces of the insulating film 102; an insulating film 112 facing the top surface and the side surfaces of the depression of the oxide semiconductor layer 106 b, the side surfaces of the oxide semiconductor layer 106 a, and the side surfaces of the insulating film 102 with the oxide semiconductor layer 107 provided therebetween; and a conductive film 104 facing the top surface and the side surfaces of the depression of the oxide semiconductor layer 106 b, the side surfaces of the oxide semiconductor layer 106 a, and the side surfaces of the insulating film 102 with the insulating film 112 and the oxide semiconductor layer 107 provided therebetween.

Note that as illustrated in the drawings, an n-type region (denoted by n) is provided between the oxide semiconductor layer 106 b, and the conductive film 116 a or the conductive film 116 b.

As illustrated in FIG. 16A, the length in the dashed-dotted line F1-F2 direction (also referred to as the channel length direction, the long-side direction, or the longitudinal direction) of the depression of the oxide semiconductor layer 106 b is referred to as L1, and the length in the dashed-dotted line F3-F4 direction (also referred to as the channel width direction, the short-side direction, or the lateral direction) of the depression of the oxide semiconductor layer 106 b is referred to as W1. The length in the dashed-dotted line F1-F2 of the projection and the depression of the oxide semiconductor layer 106 b is referred to as L2. The length in the dashed-dotted line F3-F4 direction of the projection of the oxide semiconductor layer 106 b is referred to as W2. Note that when seen from the depression of the oxide semiconductor layer 106 b, the jutting amount (also referred to as the protruding amount) of an upper portion of the projection is the same as that of a lower portion of the projection in the drawing.

As illustrated in FIG. 16B, a difference between the height of the projection of the oxide semiconductor layer 106 b and the height of the depression thereof (also referred to as the depth of the depression) is referred to as t. A difference between the height of the interface between the oxide semiconductor layer 106 a and the oxide semiconductor layer 106 b and the height of the interface between the insulating film 112 and the conductive film 104 in a region not overlapping with the oxide semiconductor layer 106 a and the oxide semiconductor layer 106 b (here corresponding to the thickness of the insulating film 112) is referred to as h. Therefore, when h is a positive value, the transistor illustrated in FIGS. 16A and 16B is regarded as having an s-channel structure.

Note that t is a difference between W2 and W1.

The description of the insulating film 402 is referred to for the insulating film 102 unless otherwise specified. The description of the oxide semiconductor layer 406 a is referred to for the oxide semiconductor layer 106 a. The description of the oxide semiconductor layer 406 b is referred to for the oxide semiconductor layer 106 b. The description of the conductive film 416 a and the conductive film 416 b is referred to for the conductive film 116 a and the conductive film 116 b. The description of the oxide semiconductor layer 407 is referred to for the oxide semiconductor layer 107. The description of the insulating film 412 is referred to for the insulating film 112. The description of the conductive film 404 is referred to for the conductive film 104.

The following table shows conditions used for the calculation.

TABLE 1 Structure L1 40 nm L2 120 nm W1 10, 20, 30, 40 nm W2 40 nm 112 Relative dielectric constant 4.1 Thickness 10 nm 107 Electron affinity 4.4 eV Eg 3.6 eV Relative dielectric constant 15 Donor density 6.60E-09 cm⁻³ Electron mobility 0.1 cm²/Vs Hole mobility 0.01 cm²/Vs Nc 5.00E+18 cm⁻³ Nv 5.00E+18 cm⁻³ Thickness 5 nm n Thickness 1 nm Donor density 5.00E+18 cm⁻³ 106b Electron affinity 4.6 eV Eg 3.2 eV Relative dielectric constant 15 Donor density 6.60E−09 cm⁻³ Electron mobility 15 cm²/Vs Hole mobility 0.01 cm²/Vs Nc 5.00E+18 cm⁻³ Nv 5.00E+18 cm⁻³ Thickness 40 nm t 0, 10, 20, 30 nm 106a Thickness 10 nm 102 Relative dielectric constant 4.1 Thickness 400 nm h 20 nm 104 Work function 5 eV 116a, 116b Work function 4.6 eV

FIGS. 17A and 17B show Vg-Id characteristics of the transistors obtained by the calculations. FIG. 17A shows the Vg-Id characteristics of the transistors at a drain voltage (Vd) of 1 V, and FIG. 17B shows the Vg-Id characteristics of the transistors at a drain voltage (Vd) of 0.1 V. The values oft are written beside curves. As described above, t is a difference between W2 and W1. W2 is 40 nm. Accordingly, when t is 0 nm, W1 is 40 nm; when t is 10 nm, W1 is 30 nm; when t is 20 nm, W1 is 20 nm; and when t is 30 nm, W1 is 10 nm.

It is found from FIGS. 17A and 17B that as t increases (W1 decreases), Vg-Id characteristics are improved, and specifically, gate voltage Vg at which drain current Id starts to rise gets closer to a positive value. Moreover, it is found that the subthreshold voltage (also referred to as S value) becomes small. That is, in comparison under conditions of the same gate voltage Vg, the drain current of the transistor in an off state becomes small.

The reason why the gate voltage Vg at which the drain current Id starts to rise gets closer to a positive value and the S value becomes small is that reduction in the channel formation region in the transistor improves the effect of an electric field of a gate electrode.

FIG. 18 is a cross-sectional view showing calculation results of current density of the oxide semiconductor layer 106 b when t is 0 nm (on the left diagram) or 30 nm (on the right diagram). A gate voltage Vg of −3 V and a drain voltage Vd of 1 V were applied.

It is found from FIG. 18 that when t is 0 nm, the current density of a region of the oxide semiconductor layer 106 b far from the conductive film 104 functioning as a gate electrode is high. On the other hand, when t is 30 nm, the current density of the entire oxide semiconductor layer 106 b is low.

These results also show that increasing t improves the effect of the gate voltage Vg, so that the drain current Id in an off-state of the transistor can be decreased.

This example shows that a transistor of one embodiment of the present invention has excellent electrical characteristics.

This application is based on Japanese Patent Application serial no. 2013-193615 filed with Japan Patent Office on Sep. 19, 2013, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a semiconductor comprising a first region, a second region, and a third region provided between the first region and the second region; a source electrode over and in contact with a top surface of the first region; a drain electrode over and in contact with a top surface of the second region; a gate insulating film over the semiconductor; and a gate electrode facing the semiconductor with the gate insulating film therebetween, the gate electrode overlapping with the third region, wherein a length of the third region is shorter than a length of the first region and the second region, in a channel width direction, wherein the semiconductor has a depression, wherein a part of a bottom surface of the gate electrode and the depression are overlapped with each other, and wherein the part of the bottom surface of the gate electrode is positioned lower than the top surface of the first region and the top surface of the second region.
 2. The semiconductor device according to claim 1, wherein a thickness of the third region is smaller than a thickness of the first region and a thickness of the second region.
 3. The semiconductor device according to claim 1, wherein the source electrode and the drain electrode cover side surfaces of the semiconductor.
 4. The semiconductor device according to claim 1, wherein the semiconductor is an oxide semiconductor.
 5. The semiconductor device according to claim 4, wherein the oxide semiconductor comprises In, Ga and Zn.
 6. The semiconductor device according to claim 1, wherein the semiconductor has a stacked-layer structure.
 7. The semiconductor device according to claim 1, further comprising: a substrate; and an insulating film over the substrate, wherein the semiconductor is provided over the insulating film.
 8. The semiconductor device according to claim 7, wherein the insulating film has a projection, and wherein the semiconductor is provided over the projection.
 9. A semiconductor device comprising: a first semiconductor; a source electrode over and in contact with a first part of the first semiconductor; a drain electrode over and in contact with a second part of the first semiconductor; a second semiconductor over the first semiconductor and the source electrode and the drain electrode; a gate insulating film over the second semiconductor; and a gate electrode facing the second semiconductor with the gate insulating film therebetween, wherein a length of a region of the first semiconductor, which is not in contact with the source electrode and the drain electrode, is shorter than a length of a region of the first semiconductor, which is in contact with the source electrode or the drain electrode, in a channel width direction.
 10. The semiconductor device according to claim 9, wherein a thickness of the region of the first semiconductor, which is not in contact with the source electrode and the drain electrode, is smaller than a thickness of the region of the first semiconductor, which is in contact with the source electrode or the drain electrode.
 11. The semiconductor device according to claim 9, wherein the source electrode and the drain electrode cover side surfaces of the first semiconductor.
 12. The semiconductor device according to claim 9, wherein the first semiconductor is a first oxide semiconductor, and wherein the second semiconductor is a second oxide semiconductor.
 13. The semiconductor device according to claim 12, wherein the first oxide semiconductor comprises In, Ga and Zn, and wherein the second oxide semiconductor comprises In, Ga and Zn.
 14. The semiconductor device according to claim 9, further comprising: a substrate; and an insulating film over the substrate, wherein the first semiconductor is provided over the insulating film.
 15. The semiconductor device according to claim 14, wherein the insulating film has a projection, and wherein the first semiconductor is provided over the projection.
 16. A method for manufacturing a semiconductor device comprising the steps of: forming a conductive film over a semiconductor; dividing the conductive film over the semiconductor to form a source electrode and a drain electrode; forming a gate insulating film over the semiconductor; and forming a gate electrode facing the semiconductor with the gate insulating film therebetween, wherein the semiconductor comprises a first region, a second region, and a third region provided between the first region and the second region, wherein the source electrode is in contact with a top surface of the first region, wherein the drain electrode is in contact with a top surface of the second region, wherein a thickness of the third region is smaller than that of the first region and the second region, wherein a length of the third region is shorter than a length of the first region and the second region, in a channel width direction, wherein the semiconductor has a depression, wherein a part of a bottom surface of the gate electrode and the depression are overlapped with each other, and wherein the part of the bottom surface of the gate electrode is positioned lower than the top surface of the first region and the top surface of the second region.
 17. The method according to claim 16, wherein the source electrode and the drain electrode cover side surfaces of the semiconductor.
 18. The method according to claim 16, wherein the semiconductor is an oxide semiconductor.
 19. The method according to claim 18, wherein the oxide semiconductor comprises In, Ga and Zn.
 20. The method according to claim 16, wherein the semiconductor has a stacked-layer structure.
 21. The method according to claim 16, wherein the semiconductor is formed over a substrate with an insulating film interposed therebetween.
 22. The method according to claim 21, wherein the insulating film has a projection, and wherein the semiconductor is provided over the projection.
 23. A semiconductor device comprising: a semiconductor comprising a first region, a second region, and a third region provided between the first region and the second region; a source electrode over and in contact with a top surface of the first region; a drain electrode over and in contact with a top surface of the second region; a gate insulating film over the semiconductor; and a gate electrode facing the semiconductor with the gate insulating film therebetween, the gate electrode overlapping with the third region, wherein a length of the third region is shorter than a length of the first region and the second region, in a channel width direction, wherein the semiconductor has a depression, wherein a part of a bottom surface of the gate electrode and the depression are overlapped with each other, and wherein a side surface of the semiconductor faces the gate electrode with the gate insulating film therebetween in the channel width direction.
 24. The semiconductor device according to claim 23, wherein a part of a bottom surface of the gate electrode is positioned lower than a bottom surface of the semiconductor.
 25. The semiconductor device according to claim 23, wherein a thickness of the third region is smaller than a thickness of the first region and a thickness of the second region.
 26. The semiconductor device according to claim 23, wherein the source electrode and the drain electrode cover side surfaces of the semiconductor.
 27. The semiconductor device according to claim 23, wherein the semiconductor is an oxide semiconductor.
 28. The semiconductor device according to claim 27, wherein the oxide semiconductor comprises In, Ga and Zn.
 29. The semiconductor device according to claim 23, wherein the semiconductor has a stacked-layer structure. 